In Boolean functions and propositional calculus, the Sheffer stroke, named after Henry M. Sheffer, written "|" (see vertical bar, not to be confused with "||" which is often used to represent disjunction), "Dpq", or "↑" (an upwards arrow), denotes a logical operation that is equivalent to the negation of the conjunction operation, expressed in ordinary language as "not both". It is also called nand ("not and") or the alternative denial, since it says in effect that at least one of its operands is false. In Boolean algebra and digital electronics it is known as the NAND operation.