Figure 6: Schematic diagram of a typical concurrent-chains procedure. In the initial-links, both keys are lighted white, and responding occasionally produces entry, according to equal concurrent VI schedules, into one of two mutually exclusive terminal- links signaled by red or green. Responding in the terminal-links produces reinforcement, after which the initial-links are reinstated. The ratio of initial-link responses is taken as a measure of preference between the terminal-link discriminated operants.