Various forms of opening and closing bracket-like delimiters are used for a number of distinct notational purposes in mathematics. The most common variants of bracket notation include:
1. angle brackets (e.g., inner product, expectation value),
2. braces (also sometimes called "curly braces" for clarity) (e.g., grouping, set delineation, piecewise delineation),
3. parentheses (e.g., grouping, function application, open interval endpoints, matrix delineation),
4. square brackets (e.g., grouping, closed interval endpoints, commutators, simple continued fractions).