The WordPress excerpt is the first part of your post, unless you use optional excerpts, and will end with […]. This is not a read more or continue reading link. The default WordPress excerpt is 55 characters.
Recently, I was working on project where the the main post page displays ads that have a very short blurb about them and then the full ad uses custom fields to display the rest of the pertinent information. One ad blurb was slightly longer than the others so we were getting the excerpt […]. Since this wasn’t happening with any other ads and was unlikely to happen very often, I decided that I wanted to make the excerpt longer so the content wouldn’t get split.