The Four CSS Rules of Multiplicity
http://www.cssnewbie.com/css-rules-multiplicity/

Glazebrook, Rob L.
CSSnewbie
2008

Abstract:

One quick and easy way to keep your CSS clean and well-structured is to remember (what I’m going to title) the four CSS Rules of Multiplicity. They are: Multiple declarations can live in a single rule. Multiple selectors can preface the same rule set. Multiple rules can be applied to the same selector. Multiple classes can be set on a single element.