Rails internationalization (I18n): Tutorial on Rails locales & more

In this tutorial, you’ll learn how to get started with Rails i18n (internationalization) to translate your application into multiple languages. We’ll cover everything you need to know: working with translations, localizing dates and times, and switching locales. Along the way, we’ll also touch on the importance of software localization for ensuring your app delivers a smooth experience for users across different regions.

To make things practical, we’ll build a sample application and enhance it step by step. By the end, you’ll have a solid understanding of Rails I18n and the confidence to implement it in real-world projects.

The source code for this tutorial can be found on GitHub.

Preparing for Rails i18n

As mentioned earlier, we’ll see all the relevant concepts in action. Let’s start by creating a new Rails application:

rails new LokaliseI18n

This tutorial uses Rails 8, but most of the concepts apply to earlier versions as well.

Generating static pages

Create a StaticPagesController with an index action for the main page::

rails g controller StaticPages index

Update the views/static_pages/index.html.erb file to include some sample content:

<h1>Welcome!</h1>

<p>We provide some fancy services to <em>good people</em>.</p>

Adding a feedback page

Let’s add a feedback page where users can share their (hopefully positive) opinions. Each piece of feedback will include the author’s name and the message. To generate the necessary files, run:

rails g scaffold Feedback author message

We’re only interested in two actions:

new: Displays a form for posting feedback and lists all existing reviews.
create: Validates and saves the new feedback.

Modify the new action to fetch all feedback from the database, ordered by creation date:

# feedbacks_controller.rb
# ... 
def new
  @feedback = Feedback.new
  @feedbacks = Feedback.order(created_at: :desc)
end

In the create action, redirect users back to the feedback page after a successful form submission:

# feedbacks_controller.rb
# ... 
def create
  @feedback = Feedback.new(feedback_params)

  if @feedback.save
    redirect_to new_feedback_path
  else
    @feedbacks = Feedback.order(created_at: :desc)
    render :new, status: :unprocessable_entity
  end
end

Views and routes

Update the new view to display all feedback:

<!-- views/feedbacks/new.html.erb -->
<!-- other code goes here... -->

<%= render @feedbacks %>

Next, tweak the partial for individual feedback entries:

<!-- views/feedbacks/_feedback.html.erb -->

<article id="<%= dom_id(feedback) %>">
  <em>
    <%= tag.time feedback.created_at, datetime: feedback.created_at %><br>
    Posted by <%= feedback.author %>
  </em>
  <p>
    <%= feedback.message %>
  </p>
  <hr>
</article>

Define the necessary routes:

# config/routes.rb

Rails.application.routes.draw do
  resources :feedbacks
  root 'static_pages#index'
end

Adding navigation and layout updates

Add a global menu to the layout for easy navigation:

<!-- views/layouts/application.html.erb -->
<!-- other code goes here... -->

<nav>
  <ul>
    <li><%= link_to 'Home', root_path %></li>
    <li><%= link_to 'Feedback', new_feedback_path %></li>
  </ul>
</nav>

Also, tweak the layout to include the charset and display the current locale:

<html lang="<%= I18n.locale %>">
  <head>
    <meta charset="utf-8">

<!-- ... other code ... ->

Final setup for Rails i18n

Run migrations and start the Rails server:

rails db:migrate
rails s

Navigate to http://localhost:3000 to ensure everything is working correctly.

Now that we have a basic app up and running, let’s proceed to the main part—localizing our application!

A bit of Rails I18n configuration

Before we start translating, we need to decide which languages our application will support. You can pick any, but for this tutorial, we’ll use English (as the default) and French. Update the config/application.rb file to reflect this:

# ...

config.i18n.available_locales = %i[en fr]
config.i18n.default_locale = :en

Also hook up a rails-i18n gem, which has locale data for different languages. For example, it has translated month names, pluralization rules, and other useful stuff.

To simplify things, we’ll use the rails-i18n gem. This gem provides locale data for various languages, such as translated month names, pluralization rules, and other helpful configurations.

Add the gem to your Gemfile:

# Gemfile
# ... 
gem 'rails-i18n', '~> 8'

Then install the library:

bundle install

That’s it! Now we have everything set up to start adding translations.

Storing translations for Rails i18n

Now that everything is configured, let’s start translating the home page content.

Localized views

The simplest way to add translations is by using localized views. This involves creating separate view files for each supported language. The naming convention is:

index.LOCALE_CODE.html.erb

Here, LOCALE_CODE corresponds to the language code. For this demo, we’ll create two views:

index.en.html.erb for English
index.fr.html.erb for French

Add the localized content to these files:

<!-- views/static_pages/index.en.html.erb -->
<h1>Welcome!</h1>
<p>We provide some fancy services to <em>good people</em>.</p>

<!-- views/static_pages/index.fr.html.erb -->
<h1>Bienvenue !</h1>
<p>Nous offrons des services fantastiques aux <em>bonnes personnes</em>.</p>

Rails will automatically render the correct view based on the currently set locale. Handy, right?

However, while localized views work well for small projects or static pages, they aren’t always practical—especially when you have a lot of dynamic content. For that, we use translation files.

Translation files for Rails i18n

Instead of hardcoding text into views, Rails allows you to store translated strings in separate YAML files under the config/locales directory. This is the recommended approach for managing translations at scale.

Open the config/locales folder, and you’ll notice a default en.yml file with some sample data:

en:
  hello: "Hello world"

Here:

en is the top-level key, representing the language code.
The hello key maps to the translated string "Hello world". Just make sure to properly organize translation keys for your own convenience.

Let’s replace the sample data with a welcome message for the home page. Update en.yml like this:

# config/locales/en.yml
en:
  welcome: "Welcome!"

Now, create a new fr.yml file in the same directory to add the French translation:

# config/locales/fr.yml
fr:
  welcome: "Bienvenue !"

With these files, you’ve successfully created translations for your first string—great!

Performing simple translations with Rails I18n

Now that we’ve populated the YAML files with some data, let’s see how to display those translated strings in the views. It’s as simple as using the translate method (aliased as t). This method requires just one argument—the name of the translation key:

<!-- views/static_pages/index.html.erb -->

<h1><%= t 'welcome' %></h1>

When the page is rendered, Rails looks up the string that corresponds to the provided key and displays it. If the translation isn’t found, Rails will render the key itself (and format it into a more human-readable form).

Naming and organizing translation keys

Translation keys can be named almost anything, but it’s always a good idea to use meaningful names that describe their purpose. Additionally, it’s best to keep your keys well-organized—you can refer to best practices in the Translation keys: naming conventions and organizing blog post for more detailed guidelines.

Adding the second message

Let’s add the next string to both en.yml and fr.yml:

# config/locales/en.yml
en:
  welcome: "Welcome!"
  services_html: "We provide some fancy services to <em>good people</em>."

# config/locales/fr.yml
fr:
  welcome: "Bienvenue!"
  services_html: "Nous fournissons des services sophistiqués aux <em>bonnes personnes</em>."

Why the `_html` postfix?

You may have noticed the _html postfix on the translation key. Here’s why:

By default, Rails will escape any HTML tags, rendering them as plain text. Since we want the <em> tag to remain intact and be displayed as formatted HTML, we mark the string as “safe HTML” using this convention.

Using the `t` method again

Now, let’s render the second message in the view:

<!-- views/static_pages/index.html.erb -->
<!-- ... --->

<p><%= t 'services_html' %></p>

And that’s it! Rails will display the proper translation values based on the current locale, and your HTML formatting will remain untouched.

More on translation keys and Rails i18n

Our homepage is now localized, but let’s pause and consider what we’ve done so far. While our translation keys have meaningful names, what happens as our application grows? For example, imagine we have 500+ messages—not an unrealistic number for even a small app. Larger websites can easily have thousands of translations.

If all the keys are stored directly under the en key with no further organization, this leads to two main problems:

Ensuring all keys have unique names becomes increasingly difficult.
It’s hard to locate related translations, such as those for a specific page or feature.

Grouping translation keys

To avoid these issues, it’s a good idea to group your keys logically. For example, you can nest your translations under arbitrary keys:

en: 
  main_page:
    header:
      welcome: "Welcoming message goes here"

There’s no limit to the nesting levels (though it’s best to keep it reasonable). This also allows you to use identical key names in different groups without any conflicts.

Following the view folder structure

A useful approach is to mirror the folder structure of your views in your YAML files. This makes your translations easy to navigate and aligns naturally with Rails conventions. Let’s reorganize our existing translations like so:

English:

# config/locales/en.yml
en:
  static_pages:
    index:
      welcome: "Welcome!"
      services_html: "We provide some fancy services to <em>good people</em>."

And French:

# config/locales/fr.yml
fr:
  static_pages:
    index:
      welcome: "Bienvenue!"
      services_html: "Nous fournissons des services sophistiqués aux <em>bonnes personnes</em>."

Referencing translations

To access these nested keys, you provide the full path to the key in the t helper methods:

<!-- views/static_pages/index.html.erb -->

<h1><%= t 'static_pages.index.welcome' %></h1>

<p><%= t 'static_pages.index.services_html' %></p>

However, Rails provides a shortcut called lazy lookup. If you are referencing translations in a view or controller, and the keys are namespaced properly (following the folder structure), you can omit the full path. Instead, use a leading dot (.):

<!-- views/static_pages/index.html.erb -->

<h1><%= t '.welcome' %></h1>

<p><%= t '.services_html' %></p>

The leading dot tells Rails to look for the translation key relative to the current namespace—in this case, static_pages.index.

Translating the global menu

Next, let’s localize our global menu and namespace these translations properly:

# config/locales/en.yml
en:
  global:
    menu:
      home: "Home"
      feedback: "Feedback"

# config/locales/fr.yml
fr:
  global:
    menu:
      home: "Page d'accueil" 
      feedback: "Retour"

Since the global menu is in the layout file, we can’t use lazy lookup here. Instead, provide the full path to the keys:

<!-- views/layouts/application.html.erb -->
<!-- ... --->
<nav>
  <ul>
    <li><%= link_to t('global.menu.home'), root_path %></li>
    <li><%= link_to t('global.menu.feedback'), new_feedback_path %></li>
  </ul>
</nav>

Translating models

Now let’s move on to the feedback page and localize the form.

Translating input labels

Rails makes it easy to translate model attributes. By providing the appropriate translations under the activerecord namespace, Rails will automatically pick them up when generating form labels.

Update your YAML files as follows:

English:

# config/locales/en.yml
en:
  activerecord:
    attributes:
      feedback:
        author: "Your name"
        message: "Message"

# config/locales/fr.yml
fr:
  activerecord:
    attributes:
      feedback:
        author: "Votre nom"
        message: "Message"

With these translations in place, Rails will automatically display the localized labels for the author and message fields.

Translating the submit button

Rails also allows you to provide translations for the model name, which are used to generate button text like “Create Feedback.” For example:

# config/locales/en.yml
en:
  activerecord:
    models:
      feedback: "Feedback"

This would make Rails render the button text as “Create Feedback”.

However, if you prefer a generic “Submit” button, you can define it in a global namespace instead:

# config/locales/en.yml
en:
  global:
    forms:
      submit: "Submit"

# config/locales/fr.yml
fr:
  global:
    forms:
      submit: "Soumettre"

Now, update the submit button in your form partial to use this translation:

<!-- views/feedbacks/_form.html.erb -->
<!-- ... --->
<%= form.submit t('global.forms.submit') %>

Error messages

We don’t want users to submit empty feedback, so let’s add some simple validations to the Feedback model:

# models/feedback.rb
# ...

validates :author, presence: true
validates :message, presence: true, length: { minimum: 5 }

Translating error messages

But what about the corresponding error messages? How do we translate them? The rails-i18n gem includes localized error messages for many languages, so you don’t need to do anything extra for common validation errors.

For example, if you’re using the French locale, Rails will automatically render the appropriate error messages. If you want to customize these default messages further, refer to the official Rails I18n guide.

One problem with the form, however, is that the error messages subtitle (the one that says “N errors prohibited this feedback from being saved:”) is not translated. Let’s fix it now and also discuss pluralization.

Pluralization rules in Rails i18n

Since the feedback form may display one or more error messages, the word “error” in the subtitle needs to be pluralized correctly. In English, this is usually done by adding an “s” (e.g., error → errors), but in other languages, pluralization rules can be more complex.

Luckily, the rails-i18n gem handles all the pluralization rules for supported languages, so you don’t have to write them yourself.

English and French pluralization

For both English and French, there are just two pluralization cases:

one – Singular (e.g., “1 error”).
other – Plural (e.g., “2 errors”).

Here’s how you define them in the translation files:

English:

# config/locales/en.yml
en:
  global:    
    forms:
      submit: "Submit"
      messages:
        errors:
          one: "One error prohibited this form from being saved:"
          other: "%{count} errors prohibited this form from being saved:"

French:

# config/locales/fr.yml
fr:
  global:    
    forms:
      submit: "Soumettre"
      messages:
        errors:
          one: "Une erreur a interdit l'enregistrement de ce formulaire :"
          other: "%{count} erreurs ont empêché la sauvegarde de ce formulaire :"

Here, %{count} is used for interpolation—Rails inserts the value of count into the string wherever %{count} appears.

Handling complex pluralization rules with Rails i18n

If you’re working with a language that has more than two plural forms, you need to provide additional keys. For example, Russian has four plural forms:

# config/locales/ru.yml
ru:
  global:
    forms:
      messages:
        errors:
          one: "Найдена одна ошибка:"
          few: "Найдены %{count} ошибки:"
          many: "Найдено %{count} ошибок:"
          other: "Найдена %{count} ошибка:"

Here’s what each key means:

one: Used for 1 (e.g., одна ошибка).
few: Used for 2–4 (e.g., две ошибки).
many: Used for 5 and higher (e.g., пять ошибок).
other: Catch-all, sometimes for 0 or edge cases.

Using translations in the form

Now let’s use these translations in the feedback form. Update the feedbacks/_form.html.erb partial like this:

<%= form_with(model: feedback) do |form| %>
  <% if feedback.errors.any? %>
    <div style="color: red">
      <h2><%= t 'global.forms.messages.errors', count: feedback.errors.count %></h2>

      <ul>
        <% feedback.errors.each do |error| %>
          <li><%= error.full_message %></li>
        <% end %>
      </ul>
    </div>
  <% end %>

  <!-- ... other code ... -->
<% end %>

The t method fetches the correct translation based on the key and the count variable.
Rails selects the appropriate plural form (one, other, few, or many) based on the count.
The %{count} placeholder is dynamically replaced with the actual number of errors.

For example:

With 1 error, the English subtitle will read: “One error prohibited this form from being saved.”
With 3 errors, it will display: “3 errors prohibited this form from being saved.”

Working with date and time

Now let’s localize the _feedback.html.erb partial. We need to translate two pieces of text:

The date and time (the created_at field).
The “Posted by…” string, which includes the author’s name.

Translating “Posted by…” with interpolation

For the “Posted by…” string, we’ll use interpolation to include the author’s name dynamically. Add the following translations to your YAML files:

# config/locales/en.yml
en:
  feedbacks:
    posted_by: "Posted by %{author}"

# config/locales/fr.yml
fr:
  feedbacks:
    posted_by: "Envoyé par %{author}"

Using translations in the view

Now, update the partial to include the t method for the “Posted by…” text:

<!-- views/feedbacks/_feedback.html.erb -->

<%= tag.article id: dom_id(feedback) do %>
  <em>
    <%= tag.time feedback.created_at, datetime: feedback.created_at %><br>
    <%= t 'feedbacks.posted_by', author: feedback.author %>
  </em>
  <p>
    <%= feedback.message %>
  </p>
  <hr>
<% end %>

Here, the t method fetches the appropriate translation and interpolates the author value dynamically.

Localizing date and time

To handle the created_at timestamp, we’ll use the localize method (aliased as l). This works similarly to Ruby’s strftime but automatically uses translated month and day names based on the current locale.

Let’s use the predefined :long format:

<%= tag.article id: dom_id(feedback) do %>
  <em>
    <%= tag.time l(feedback.created_at, format: :long), datetime: feedback.created_at %><br>
    <%= t 'feedbacks.posted_by', author: feedback.author %>
  </em>
  <p>
    <%= feedback.message %>
  </p>
  <hr>
<% end %>

With the l method:

Rails automatically uses the current locale to determine the translation for month names, days, etc.
The :long format is a built-in Rails format that provides a user-friendly, full-length date.

For example:

English: “March 15, 2024 12:30 PM”
French: “15 mars 2024 12:30”

Defining custom date and time formats

If you need a specific date or time format, you can define your own. Add a custom format under the time.formats namespace:

# config/locales/en.yml
en:
  time:
    formats:
      custom_short: "%H:%M"

You can then use it in your view like this:

<%= l(feedback.created_at, format: :custom_short) %>

This would render only the time, e.g., “12:30”.

Switching between locales in Rails i18n

Now that our app is fully translated, there’s just one problem: we can’t change the locale. While it might sound minor, it’s actually a major usability issue—so let’s fix it!

We’ll implement the following solution:

URLs will include an optional :locale parameter, like: http://localhost:3000/en/some_page.
If this parameter is set and matches a supported locale, Rails will switch to that language.
If no parameter is provided or the locale isn’t supported, Rails will fall back to the default locale (English).

Sounds simple? Let’s dive into the code.

Routes

First, update the config/routes.rb file to include a locale scope:

# config/routes.rb
scope "(:locale)", locale: /#{I18n.available_locales.join("|")}/ do
  # your routes here...
  resources :feedbacks
  root 'static_pages#index'
end

Here:

The (:locale) scope makes the :locale parameter optional.
We validate the locale using a regular expression to ensure it matches one of the supported locales.

Adding a concern for locale handling

Next, create an internationalization concern to handle locale switching.

Include the concern in ApplicationController:

# controllers/application_controller.rb

class ApplicationController < ActionController::Base
  include Internationalization
end

Create this new concern:

# controllers/concerns/internationalization.rb
module Internationalization
  extend ActiveSupport::Concern

  included do
    around_action :switch_locale

    private

    def switch_locale(&action)
      locale = locale_from_url || locale_from_headers || I18n.default_locale
      response.set_header 'Content-Language', locale
      I18n.with_locale(locale, &action)
    end

    def locale_from_url
      locale = params[:locale]
      return locale if I18n.available_locales.map(&:to_s).include?(locale)
    end

    def locale_from_headers
      header = request.env['HTTP_ACCEPT_LANGUAGE']
      return if header.nil?

      locales = parse_header(header)
      return if locales.empty?

      detect_from_available(locales)
    end

    def parse_header(header)
      header.gsub(/\s+/, '').split(',').map do |tag|
        locale, quality = tag.split(/;q=/i)
        quality = quality ? quality.to_f : 1.0
        [locale, quality]
      end.reject { |(locale, quality)| locale == '*' || quality.zero? }
         .sort_by { |(_, quality)| quality }
         .map(&:first)
    end

    def detect_from_available(locales)
      locales.reverse.find { |l| I18n.available_locales.any? { |al| match?(al, l) } }
    end

    def match?(str1, str2)
      str1.to_s.casecmp(str2.to_s).zero?
    end

    def default_url_options
      { locale: I18n.locale }
    end
  end
end

How it works

switch_locale: Determines the locale by checking:
- URL parameters (locale_from_url).
- Request headers (locale_from_headers).
- Defaults to I18n.default_locale if none are found.
default_url_options: Ensures all Rails URL helpers include the :locale parameter automatically.

The parse_header method handles the complexity of parsing the HTTP_ACCEPT_LANGUAGE header, which browsers send to indicate preferred languages.

Adding locale switching links

Now, let’s allow users to switch locales from the UI. Update the global menu in application.html.erb:

<!-- views/layouts/application.html.erb -->
<ul>
  <% I18n.available_locales.each do |locale| %>
    <li>
      <% if I18n.locale == locale %>
        <%= t(locale, scope: 'locales') %>
      <% else %>
        <%= link_to t(locale, scope: 'locales'), url_for(locale: locale) %>
      <% end %>
    </li>
  <% end %>
</ul>

Here:

I18n.available_locales provides a list of supported locales.
t(locale, scope: 'locales') fetches the translated name of the locale.
url_for(locale: locale) generates a URL with the updated :locale parameter.

Adding locale translations

Add translations for the locale names:

# config/locales/en.yml
en:
  locales:
    en: "English"
    fr: "French"

And French:

# config/locales/fr.yml
fr:
  locales:
    en: "Anglais"
    fr: "Français"

Testing Rails i18n out

Restart your Rails server.
Visit URLs like:
- http://localhost:3000/en → App will render in English.
- http://localhost:3000/fr → App will render in French.
Switch between locales using the links in the navigation menu.

Rails will:

Remember the locale in URLs.
Automatically include the current locale in all generated links.
Fallback to the default locale if no valid :locale is provided.

Simplify your Rails i18n workflow with Lokalise

By now, you might be thinking that supporting multiple languages on a large website is a pain. Honestly? You’re right. While Rails makes it easier with namespaced keys and multiple YAML files, it’s still on you to ensure every key is translated across all locales.

Fortunately, there’s a better way: Lokalise—a platform that simplifies managing and editing localization files. Let’s walk through the setup.

Initial Lokalise setup

Get started: Sign up for a free trial on Lokalise.
Generate your API token:
- Go to your personal profile → API tokens → generate a read/write token.
Create a new project:
- Go to Lokalise and create a “Web and mobile” project.
- Name it anything you like, set English as the base language, and add French (or your target language).
Find your project ID: On the project page, click “More” → “Settings.” The project ID will be listed here.
Use Lokalise CLI tool to facilitate file exchange.

Using the `lokalise_rails` gem

Prefer a more Rails-like experience instead of relying on command-line interface? Enter the lokalise_rails gem, which integrates Lokalise directly into your app.

Installation

Add the gem to your Gemfile:

gem 'lokalise_rails'

Then run:

bundle install
rails g lokalise_rails:install

Configuration

Update config/lokalise_rails.rb with your token and project ID:

require 'lokalise_rails'

LokaliseRails::GlobalConfig.config do |c|
  c.api_token = ENV['LOKALISE_API_TOKEN']
  c.project_id = ENV['LOKALISE_PROJECT_ID']

  # ... other options ...
end

Here you’ll need to enter your Lokalise API and the project ID.

The import and export options have sensible defaults, but you may tweak them further as necessary.

Exporting and importing translations

Use these commands to manage translations:

Export to Lokalise:

rails lokalise_rails:export

Import back into your app:

rails lokalise_rails:import

Advanced: Managing multiple directories in Rails i18n

If your app uses custom directories for translations, you can leverage the LokaliseManager gem to handle this programmatically. For example:

require 'rake'
require 'lokalise_rails'
require "#{LokaliseRails::Utils.root}/config/lokalise_rails"

namespace :lokalise_custom do
  task :export do
    # importing from the default directory (./config/locales/)
    exporter = LokaliseManager.exporter({}, LokaliseRails::GlobalConfig)
    exporter.export!

    # importing from the custom directory
    exporter = LokaliseManager.exporter({locales_path: "#{Rails.root}/config/custom_locales"}, LokaliseRails::GlobalConfig)
    exporter.export!
  rescue StandardError => e
    abort e.inspect
  end
end

This script uploads all YAML files from both config/locales and config/custom_locales.

Conclusion to Rails i18n

In this article, we’ve explored how to:

Set up Rails I18n for a multilingual app.
Use translation files and localized views.
Translate error messages, model attributes, and dynamic content.
Switch between locales with URL-based persistence.
Simplify your workflow with Lokalise and its tools.

While Rails I18n is powerful, tools like Lokalise make managing translations at scale a breeze. For further details, I recommend checking out the official Rails I18n guide.

Thank you for staying with me, and until next time!

Preparing for Rails i18n

Generating static pages

Adding a feedback page

Views and routes

Adding navigation and layout updates

Final setup for Rails i18n

A bit of Rails I18n configuration

Storing translations for Rails i18n

Localized views

Translation files for Rails i18n

Performing simple translations with Rails I18n

Naming and organizing translation keys

Adding the second message

Why the _html postfix?

Using the t method again

More on translation keys and Rails i18n

Grouping translation keys

Following the view folder structure

Referencing translations

Translating models

Translating input labels

Translating the submit button

Error messages

Translating error messages

Pluralization rules in Rails i18n

English and French pluralization

Handling complex pluralization rules with Rails i18n

Using translations in the form

Working with date and time

Translating “Posted by…” with interpolation

Using translations in the view

Localizing date and time

Defining custom date and time formats

Switching between locales in Rails i18n

Routes

Adding a concern for locale handling

How it works

Adding locale switching links

Adding locale translations

Testing Rails i18n out

Simplify your Rails i18n workflow with Lokalise

Initial Lokalise setup

Using the lokalise_rails gem

Installation

Configuration

Exporting and importing translations

Advanced: Managing multiple directories in Rails i18n

Conclusion to Rails i18n

Further reading on Rails i18n

Want the latest scoop on localization and global growth?

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