  We introduce and study twist vertex operators for a (lower-bounded
generalized) twisted modules for a grading-restricted vertex (super)algebra. We
prove duality, weak associativity, a Jacobi identity, a generalized commutator
formula, generalized weak commutativity, and convergence and commutativity for
products of more than two operators involving twist vertex operators. These
properties of twist vertex operators play an important role in the author's
recent general, direct and explicit construction of (lower-bounded generalized)
twisted modules.
