An abbreviatory convention of rule-based generative phonology which collapses two rules which differ in the order of the elements of their environment. For example, suppose a language has two rules which state that the segment X is deleted (1) before segment Y and (2) after segment Y, so the two environments are /__Y and /Y__. The convention collapses these rules into one: X → 0 // Y, where the // indicates that this is a mirror-image rule.