The lowercase Greek letter λ (lambda) is an unofficial symbol of the field of programming language theory. This usage derives from the lambda calculus, a model of computation introduced by Alonzo Church in the 1930s and widely used by programming language researchers. It graces the cover of the classic text Structure and Interpretation of Computer Programs, and the title of the so-called Lambda Papers, written by Gerald Jay Sussman and Guy Steele, the developers of the Scheme programming language.