The set of all points on a straight line between two fixed points (excluding the given two points) is represented by (a, b) (excluding the two endpoints A and B). The essence of an open interval is still a set of numbers, which is represented by symbols (a, b), generally referring to all real numbers between real numbers A and B, but excluding A and B ... which is equivalent to {x | A.

A closed interval is a connected closed set on a straight line, a set of all points (including two given points) between two fixed points on a straight line, and a number between 3 (inclusive) and 5 (inclusive).