The Okubo–Zweig–Iizuka rule or OZI rule is a consequence of quantum chromodynamics (QCD) that explains why certain decay modes appear less frequently than otherwise might be expected. It was independently proposed by Susumu Okubo, George Zweig and Jugoro Iizuka in the 1960s. It states that any strongly occurring process will be suppressed if, through only the removal of internal gluon lines, its Feynman diagram can be separated into two disconnected diagrams: one containing all of the initial-state particles and one containing all of the final-state particles.