Concept coined by Cleve Backster. A zone is a 20- to 35-second block of polygraph chart time initiated by a question having a unique psychological focusing appeal to a predictable group of examinees. In his ZCT, Backster used color-coding to identify the three zones in the ZCT: red, green, and black. Respectively, the red zone for relevant questions, the green zone for comparison questions, and the black zone for symptomatic questions.