Borcherds is best known for his resolution in 1992 of the Conway-Norton monstrous moonshine conjecture, which describes an intricate relation between the monster group and modular functions on the complex upper half-plane. To prove this conjecture, he drew upon theories that he had previously introduced, namely those of vertex algebras and Borcherds–Kac–Moody algebras, together with techniques of string theory, and applied them to the "moonshine module", a vertex operator algebra with monster symmetry constructed by Igor Frenkel, James Lepowsky, and Arne Meurman. Additional work in moonshine concerned mod "p" variants of this conjecture, and were known as modular moonshine.