The PLUGE signal was designed for rapid and accurate adjustment of the black level, reference and, hence, the luminance range, display. It provides adjacent vertical bars, one at black level, reference and continuous bars slightly above and slightly below that reference. Following initial development by the BBC, CCIR now recognizes at least eight versions. SMPTE EG 1-1990 includes a variant in which the black level, reference is flanked by bars at –4 IRE and +4 IRE. When the –4 IRE merges into the black level, reference bar, but the +4 IRE bar is distinguishable, black level, reference is correctly set. A white patch is included at peak white, to define IRE 100, and the luminance range, display CRT.