All non-WYSIWYG wikis use hyphens (----) to denote horizontal rules. Most wikis require four.
Some wikis vary the thickness of the line depending on the number of hyphens, but this is rarely used and hard to notice, so we left it out of the standard.
Q: Does this mean Creole Wikis should be intolerant to more than four hyphens? (E.g., "-----" would then just be formatted into <p>-----</p> and so still look almost like a horizontal ruler.)
A: No, we don't care: If it's four or more hyphens, it's a horizontal rule -- ChristophSauer, 2007-Jun-20 10:07 (CEST)