Although many different metasyntaxes are possible, Backus-Naur form (BNF) and Extended Backus-Naur form (EBNF), which express syntax as a set of derivation rules, are almost universally used. The variables in these metasyntaxes are properly known as metasyntactic variables, although the term is used informally in other ways.
See also: Attribute grammar, Two-level grammar