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languages employ the idea of subtyping (also called subtype polymorphism or inclusion polymorphism) to restrict the range of types that can be used in a particular case of polymorphism. In these languages, subtyping allows a function to be written to take an object of a certain type T, but also work correctly if passed an object that belongs to a type S that is a subtype of T (according to the Liskov substitution principle). This type relation is sometimes written ''S'' <: ''T''. Conversely, T is said to be a supertype of S, written ''T'' :> ''S''. Subtype polymorphism is usually resolved dynamically (see below).

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