A WordPress post can be forced to have a teaser by inserting the HTML markup
<!-- more --> after the desired teaser in the post. WordPress then splits this into the teaser and the main content. When an explicit excerpt for a post is not specified, this teaser is used as an excerpt.
more global variable is used to indicate whether the entire page of the post is to be displayed or only the teaser. Teasers can be included in every page of the post. It is set in the function
setup_postdata($post) defined in [wordpress root folder]/wp-includes/functions.php.
more global variable internally to determine whether only the teaser has to be displayed or the entire page of the post. Whether the teaser has to be displayed when the entire page of the post is displayed or not can also be specified by including the HTML markup
<!-- noteaser --> in the post. Plugin and theme developers can use this global variable if a different behavior is desired.
Back to full list of global variables.