> ## Documentation Index
> Fetch the complete documentation index at: https://engineering.trewknowledge.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Taxonomies

As we have seen, posts always appear in collections. There's often the need to create sub-collections out of this main collection. This is where taxonomies come in.

## Flat vs. Hierarchical Taxonomies

Taxonomies can be divided into main types: **hierarchical** taxonomies and **flat** taxonomies. Which one to use depends a lot on the individual scenario.

Imagine that you have news articles related cities and states/provinces. Your news article relates to a particular city and State/Province. Therefore a straightforward approach might be to have a single taxonomy with parent terms representing the States/Provinces, and child terms for the individual cities. With this approach, it would be easy to list all the news in a particular place.

As for the flat taxonomies, imagine that you have a post type for videos. You want to be able to tag these videos with the names of the people appearing in them. In this case, a flat taxonomy would be the best approach.

<Info>
  For clarification purposes, taxonomies and terms are two different things. Terms are each individual item within a taxonomy. For example, "New York" is a term within the "Locations" taxonomy.
</Info>

## Creating Taxonomies

There are two ways you can create taxonomies. You can either add the `register_taxonomy()` to the post type plugin in case the taxonomy and the post type are related, or you can create a separate plugin for the taxonomy in case multiple post types will share it.

<Info>
  Please remember to replace `PREFIX` with your own project prefix and replace `textdomain` with your own text domain.
</Info>

<Tip>
  Documentation: [https://developer.wordpress.org/reference/functions/register\_taxonomy/](https://developer.wordpress.org/reference/functions/register_taxonomy/)
</Tip>

```php PluginName.php theme={null}
<?php
/**
 * Plugin Name: PREFIX Taxonomies - Locations
 * Plugin URI: http://trewknowledge.com/
 * Description: A plugin to register the Locations taxonomy.
 * Version: 1.0.0
 * Author: Trew Knowledge
 * Author URI: http://trewknowledge.com/
 * License: GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 * Text Domain: textdomain
 */

add_action( 'init', function() {

  /* 
    Load Load Translations
  */

  load_plugin_textdomain( 'textdomain', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );

  /*
    Register Locations Taxonomy
  */

  $labels = array(
    'name'              => _x( 'Themes', 'taxonomy general name', 'textdomain' ),
    'singular_name'     => _x( 'Theme', 'taxonomy singular name', 'textdomain' ),
    'search_items'      => __( 'Search Themes', 'textdomain' ),
    'all_items'         => __( 'All Themes', 'textdomain' ),
    'parent_item'       => __( 'Parent Theme', 'textdomain' ),
    'parent_item_colon' => __( 'Parent Theme:', 'textdomain' ),
    'edit_item'         => __( 'Edit Theme', 'textdomain' ),
    'update_item'       => __( 'Update Theme', 'textdomain' ),
    'add_new_item'      => __( 'Add New Theme', 'textdomain' ),
    'new_item_name'     => __( 'New Theme Name', 'textdomain' ),
    'menu_name'         => __( 'Theme', 'textdomain' ),
  );

  $args = array(
    'hierarchical'      => true,
    'labels'            => $labels,
    'show_ui'           => true,
    'show_admin_column' => true,
    'query_var'         => true,
    'rewrite'           => array( 'slug' => 'locations' ),
    'show_in_rest'       => true,
  );

  register_taxonomy( 'prefix_tax_location', array( 'prefix_post_type' ), $args );

} );
```