In his book, The Grammar of Words: An Introduction to Linguistic Morphology, Geert Booij tells us that many English dictionaries do not present different entries for related words such as walks, walked, and walking.

He explains that this occurs because those dictionaries assume that the language user does not need this information; that the user is naturally able to construct these different forms of the word walk by applying relevant rules.

These rules for processing different forms of a lexeme are called rules of