Given a class of algebraic structures of the same signature, we can define the notions of homomorphism, subalgebra, and product. Garrett Birkhoff proved that a class of algebraic structures of the same signature is a variety if and only if it is closed under the taking of homomorphic images, subalgebras and arbitrary products. This is a result of fundamental importance to universal algebra and known as "Birkhoff's theorem" or as the "HSP theorem". "H", "S", and "P" stand, respectively, for the operations of homomorphism, subalgebra, and product.