Intervals can be expressed using several notations.  Two common forms are

Inequalities

These are defined in terms of the signs used to describe the relationship of a number with the bounds of the interval.

This interprets as x is greater than or equal to the lower bound and less than or equal to the upper bound. Different combinations of less than, less than or equals, greater than or equals or greater than signs can be used to define open or closed intervals.

Brackets

This form uses conventions associated with curved and square brackets, whilst not as obvious as the inequality notation, they do not reference a variable and are simpler to produce on an ordinary keyboard: