Some sources define silicon-controlled rectifiers and thyristors as synonymous,[5] other sources define silicon-controlled rectifiers as a proper subset of the set of thyristors, those being devices with at least four layers of alternating n- and p-type material.[6][7] According to Bill Gutzwiller, the terms "SCR" and "controlled rectifier" were earlier, and "thyristor" was applied later, as usage of the device spread internationally.[8]