Intervals are often written as a pair of numbers between two brackets, with the brackets determining whether the interval is open or closed at either end.
One convention is to use parentheses for 'open' and square brackets for closed. The interval between \(a\) and \(b\) which contains \(a\) but not \(b\) is written \([a,b)\). This conflicts with quite a few things because parentheses are overused.
Another convention is to always use square brackets, but reversed for 'open'. The interval above is written \([a, b[\).