Rules are like little if-then clauses, existing inside rule sets, that test a pattern against an address and change the address if the two match. The process of converting one form of an address into another is called rewriting. Most rewriting requires a sequence of many rules because an individual rule is relatively limited in what it can do. This need for many rules, combined with the sendmail program’s need for succinct expressions, can make sequences of rules dauntingly cryptic.
In this chapter we dissect the components of individual rules. In the next chapter we will show how groups of rules can be combined to perform necessary tasks.
 Actually they can either be if-then or while-do clauses, but we gloss over that complexity for the moment.