If A = {2, 4, 6} and B = {1, 2, 3, 4, 5, 6}, then A is a subset of B. We could write this as A ⊆ B, or if we wanted to make the point that it is a proper subset, we could write it as A ⊂ B In mathematics, a set is a collection of distinct objects, considered as an object in its own right. For example, the numbers 2, 4, and 6 are distinct objects when considered separately, but when they are considered collectively they form a single set of size three, written {2,4,6}. Sets are one of the most fundamental concepts in mathematics. Developed at the end of the 19th century, set theory is now a ubiquitous part of mathematics, and can be used as a foundation from which nearly all of mathematics can be derived.