Order theory is a branch of mathematics which investigates the properties and structure of partial orders and quasi-orders (or preorders).

These sort of orders appear naturally in the mathematical universe; such as the $\subseteq$ relation, or $\leq$ on the integers. It follows that these orders (and quasi-orders) appear in our lives as well, e.g. "which item is more expensive?"

Some mathematical concepts related to order theory are:

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