WordPress at a glance
function is not described

adjacent_posts_rel_link_wp_head() WP 3.0.0

Displays relational links for the posts adjacent to the current post for single post pages.

This is meant to be attached to actions like wp_head. Do not call this directly in plugins or theme templates.

No Hooks.

Return

null. Nothing.

Usage

adjacent_posts_rel_link_wp_head();

Notes

Changelog

Since 3.0.0 Introduced.
Since 5.6.0 No longer used in core.

Code of adjacent_posts_rel_link_wp_head() WP 5.7.2

<?php
function adjacent_posts_rel_link_wp_head() {
	if ( ! is_single() || is_attachment() ) {
		return;
	}
	adjacent_posts_rel_link();
}