Where

`a` and

`b` are

real numbers, the set:

{`x` in **R** : `a` <= `x` < `b`}

is called a

*half-open interval* on the

real line, and may be written

[`a`, `b`).

Unlike an open interval, where neither the infimum nor the supremum are members of the set, and a closed interval, where both are, here the infimum is a member and the supremum is not.