How to create custom taxonomies (register_taxonomy)

“Season,” “Brand,” “Service area”—custom taxonomies classify posts along axes of your own choosing. You get the same mechanism as categories and tags, under whatever name you like.

function my_register_taxonomies() {
  register_taxonomy('season', ['shohin'], [
    'label'             => 'Season',
    'public'            => true,
    'hierarchical'      => true,    // true = category-style / false = tag-style
    'show_admin_column' => true,    // add a column to the admin list
    'show_in_rest'      => true,    // use it in the block editor
  ]);
}
add_action('init', 'my_register_taxonomies');

The first argument is the taxonomy name; the second is the post types to attach it to (an array lets you name several). That alone adds a “Season” selector to the product editing screen.

Category-style and tag-style

One line—hierarchical—changes its character entirely.

“I want people to pick from a predetermined set” means category-style; “I want writers to add freely” means tag-style. Choose based on how the people running the site will work.

Argument cheat sheet

Archive pages and templates

Registering a taxonomy automatically creates an archive page per term.

/season/winter/     ← a list of posts belonging to "Winter"

The template responsible for that page is looked for in this order.

1. taxonomy-season-winter.php (that specific term only)
2. taxonomy-season.php (the whole taxonomy)
3. taxonomy.php
4. archive.php
5. index.php

archive.php stands in even without a dedicated file, so confirm it works first and add files as you need them.

You can output the term’s name and description in the template like this.

<h1><?php single_term_title(); ?></h1>
<?php if ($desc = term_description()) : ?>
  <div class="term-desc"><?php echo wp_kses_post($desc); ?></div>
<?php endif; ?>

Showing a post’s terms

Output the terms attached to a post with get_the_terms() (or the_terms() if you want the HTML too).

<?php the_terms(get_the_ID(), 'season', 'Season: ', ', '); ?>

To build a list of the classification itself (a filter menu), use get_terms().

Fetching only that term’s posts

When you’re building a list yourself, filter with tax_query.

$query = new WP_Query([
  'post_type' => 'shohin',
  'tax_query' => [
    ['taxonomy' => 'season', 'field' => 'slug', 'terms' => 'winter'],
  ],
]);

Choosing between taxonomies and custom fields

Both are “extra information,” but their roles differ.

• Taxonomy—an axis for filtering lists. You get a dedicated archive page and URL, and filtering is fast
• Custom field—a value displayed within the article. Price, size, date and so on

The rule of thumb is “do I want to browse a list by this data?” “I want to see only winter products” means a taxonomy; “I just want to display a price” means a custom field. It also makes a speed difference as content grows.

Naming rules and cautions

• Lowercase alphanumerics and underscores, up to 32 characters
• Avoid names WordPress uses—category, post_tag, type, author, name, date, terms and others
• Don’t use the same name as a post type (the URLs collide)

Attaching it to a post type later

To use an existing taxonomy on another post type as well, one line does it.

add_action('init', function () {
  register_taxonomy_for_object_type('season', 'post');   // use "Season" on posts too
});

Conversely, to use the standard categories and tags on a custom post type, 'taxonomies' => ['category', 'post_tag'] in register_post_type() is the easy route.

Summary

1. Just run register_taxonomy('name', ['post type'], [...]) on the init hook
2. hierarchical decides category-style (pick from a set) or tag-style (add freely)
3. taxonomy-<name>.php handles the archive. Without it, archive.php stands in
4. Axes you want to filter by are taxonomies; values you only display are custom fields

Once you can design classifications, the same article data can be presented in many different ways.