The letter of the law and the spirit of the law are two possible ways to regard rules, or laws. To obey the letter of the law is to follow the literal reading of the words of the law, whereas following the spirit of the law means enacting the intent behind the law. Although it is usual to follow both the letter and the spirit, the two are commonly referenced when they are in opposition. "Law" originally referred to legislative statute, but in the idiom may refer to any kind of rule.

Intentionally following the letter of the law but not the spirit may be accomplished through exploiting technicalities, loopholes, and ambiguous language. Rules as written (RAW) versus rules as intended (RAI) is a similar expression originating from the tabletop role-playing game community.