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.	0or x>0
Negative Numbers	A real number x is said to be negative if it is less than 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.	- ∞or x≤ 0
Non-Negative Numbers	A real number x is said to be non-negative if it is greater than or equal to 0.	0≤ x<∞ or x≥ 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.