An affix grammar is a kind of formal grammar; it is used to describe the syntax of languages, mainly computer languages, using an approach based on how natural language is typically described. The grammatical rules of an affix grammar are those of a context-free grammar, except that certain parts in the nonterminals (the affixes) are used as arguments. If the same affix occurs multiple times in a rule, its value must agree, i.e. it must be the same everywhere. In some types of affix grammar, more complex relationships between affix values are possible.