Part of the description of any real number is its sign — it can be positive, negative, or zero. When discussing a set of numbers, it can sometimes be helpful to know how to describe the sign of all numbers within the set.

	Definition	Representation
Positive Numbers	A real number $x$ is said to be positive if it is greater than $0.$	$0<x<\infty$ or $x>0$
Negative Numbers	A real number $x$ is said to be negative if it is less than $0.$	$\text{-}\infty <x<0$ or $x<0$
Non-Positive Numbers	A real number $x$ is said to be non-positive if it is less than or equal to $0.$	$\text{-}\infty <x\le 0$ or $x\le 0$
Non-Negative Numbers	A real number $x$ is said to be non-negative if it is greater than or equal to $0.$	$0\le x<\infty$ or $x\ge 0$

Each of these sets of numbers — positive, negative, non-positive, and non-negative — can be represented on a number line. The point at $0$ is closed when it is included in the set and open when it is not.