A train-of-four (TOF) consists of four equal stimuli delivered at 0.5-s intervals. The TOF ratio is calculated by comparing the magnitude of the fourth evoked response or twitch (T₄) to that of the first response (T₁). During partial nondepolarizing block, T₄ decreases preferentially, such that there is fade (i.e., TOF ratio less than 1.0). In the illustration above, the TOF ratio = 0.40 (40%).