# System F-sub

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In the branch of mathematical logic known as type theory, System F<:, pronounced "F-sub", is an extension of system F with subtyping. System F<: has been of central importance to programming language theory since the 1980s[citation needed] because the core of functional programming languages, like those in the ML family, support both parametric polymorphism and record subtyping, which can be expressed in System F<:.