An interval on the real line is the set of all numbers that fall between two specified endpoints.

Let a and b be real numbers with a < b. We can have the following types of finite intervals:

• The open interval (a,b) is the set of all real numbers x that fall strictly in between a and b. That is, all real numbers x with a < x < b. The values a and b are not included in this interval.

• The closed interval [a,b] is the set of all real numbers x that fall (non-strictly) in between a and b. That is, all real numbers x with a ≤ x ≤ b. The values a and b are included in this interval.

• The interval [a,b) is the set of all real numbers x with a ≤ x < b. a is included in this interval, while b is not.

• The interval (a,b] is the set of all real numbers x with a < x ≤ b. b is included in this interval, while a is not.

The values a and b are called endpoints because they're the points at either end of the interval.

To remember which of the two intervals (a,b) or [a,b] includes the endpoints a and b, try thinking of the interval notation like arms. If the arms are like the brackets [a,b] then they are holding a and b firmly in there. If arms are like the parentheses (a,b), then the endpoints a and b slip out.

The interval (a,b) is called open, while the interval [a,b] is called closed. The intervals [a,b) and (a,b] are neither open nor closed. We might hear these intervals called "half-closed," "semi-closed," "half-open", or "semi-open".

We can also have infinite intervals. Since ∞ isn't a number, we'll always have parentheses around ∞ or ∞, not the closed brackets. Here are the types of infinite intervals we can have,  assuming that a is some finite number:

• (-∞, ∞), which is the whole real line. In other words, this is the set of all real numbers.
    
• (-∞,a) is the set of real numbers x with x < a. This interval does not include a.
    
• (-∞,a] is the set of real numbers x with x ≤ a. This interval does include a.
    
• (a,∞) is the set of real numbers x with a < x. This interval does not include a.
    
• [a,∞) is the set of real numbers x with a ≤ x. This interval does include a.

The infinite intervals (-∞, ∞), (-∞, a], and [a,∞) are closed intervals. The infinite intervals (-∞,a) and (a,∞) are open.

The infinite intervals (-∞, ∞), (-∞, a], and [a,∞) are closed intervals. The infinite intervals (-∞,a) and (a,∞) are open.