ordered pair

An ordered pair $(a,b)$ is a pair of objects in which their order is significant, i.e. it is not necessary that $(a,b)=(b,a)$.

Ordered pairs are also called 2-tuples, or sequences of length 2.

An unordered pair $\{a,b\}$ is a set with 2 elements in which order is not significant, i.e. it is always true that $\{a,b\}=\{b,a\}$.