nLab thin category

Thin categories

Thin categories


A thin or posetal category a category in which, given any two objects xx and yy and any two morphisms ff and gg from xx to yy, the morphisms ff and gg are equal:

xgfyf=g x \underoverset{\quad g \quad}{f}{\rightrightarrows} y \;\;\;\implies\;\;\; f = g


Up to isomorphism, a thin category is the same thing as a proset (a preordered set). Up to equivalence, a thin category is the same thing as a poset. So mostly we just talk about posets here, but some references want to distinguish these from thin categories. (It is really a question of whether you're working with strict categories, which are classified up to isomorphism, or categories as such, which are classified up to equivalence.)


A poset is a thin category. In particular so are (semi)lattices, Heyting algebras and frames.

Last revised on May 3, 2022 at 15:41:28. See the history of this page for a list of all contributions to it.