A "coefficient" is a numerical value, or letter representing a numerical constant, that multiplies a variable (the operator is omitted). A "term" is an addend or a summand, a group of coefficients, variables, constants and exponents that may be separated from the other terms by the plus and minus operators. Letters represent variables and constants. By convention, letters at the beginning of the alphabet (e.g. formula_2) are typically used to represent constants, and those toward the end of the alphabet (e.g. formula_3 and ) are used to represent variables. They are usually written in italics.