The symbols used to enclose a group of symbols or numbers that are to be taken together. Brackets can be used to change the order in which operations are to be done. For example, 7×2 + 3 will be 14 + 3 = 17. If the sum intended is 7×5 = 35 then brackets can be put round 2 + 3 so that the sum is taken before the product, i.e. 7×(2 + 3). Brackets may appear as round brackets or parentheses (…), as square brackets […], or as braces {…}, commonly used in set-theoretic notation. Brackets can be nested so that the whole contents of the inner bracket are treated as a single term in the larger bracket. The number of ways n pairs of brackets can be nested equals the nth Catalan number. See also BIDMAS.