A closed interval is one that includes its endpoints: for example, the set .
To write this interval in interval notation, we use closed brackets [ ]:
An open interval is one that does not include its endpoints, for example, .
To write this interval in interval notation, use parentheses :
You can also have intervals which are half-open and half-closed:
You can also use interval notation together with the set union operator to write subsets of the number line made up of more than one interval: