link to Senate Rules. Please note what is commonly referred to as "Rules" are actually called Chapters in the document:
http://www.senate.gov/legislative/common/briefing/Standing_Rules_Senate.htm#21
The official version of the rules printed by the US Government printing office does not call them chapters. The Government printing office publication refers to them as rules and lists the rule number before each rule using roman numerals preceded by the word Rule. The copy of the rules posted on the Senate Rules Committee site also refers to them by Rule numbers and uses roman numerals and the word Rule.
The official printed version has an index as well as a table of contents and rule numbers not chapters. Some web listings call the table of contents by the name Index.
Chapters and Index are likely made up terms adopted by the interns that posted the rules on the web.