The purpose of the present investigation was to make an accurate and thorough determination of the constant, or time-order errors (C.E.'s), which occur in comparing short tonal durations (intervals of time filled with continuous tone). These errors, as well as the location of the indifference duration, i.e., the duration at which time-order errors disappear, were studied under a considerable variety of carefully controlled conditions, with a total of five hundred and twentyfour subjects from whom no less than 99,480 judgments were obtained. The durations used in the investigation ranged from 0.2 to 36.0 sec. Most of the work, however, was concentrated upon those between 0.5 and 2.0 sec.The preceding experience of the subject in comparing durations was revealed by the present study as an enormously important factor in determining the time-order errors and the indifference duration. This experience may consist of previous sittings, or of earlier trials of a given sitting. Previous sittings are particularly influential in the case of a long series of sittings with a wide range of standards. The earlier trials at a given sitting, under certain conditions, also result in a marked change in the constant errors (and consequent shift in the indifference point) shown by later trials; a change which readily comes to light in long sittings when the data are fractionated for successive portions of the sitting.This factor of experience is one which has been very largely neglected in the past. In the present study it is given credit Ri, relay which always controlled the sound making device. Rt, shunt relay. Ri, relay which served to break circuit which activated Ri. B. The electro-mechanical shutter apparatus for presenting sounds. M, electric magnets which operated the shutter. G, aluminum shutter attached to shaft with lever. T, glass tubes which conducted the sound to subjects' room. Ph., phone. A, audio oscillator. C. Set-up for timing the shutter. G, shutter. H, Hipp chronoscope. M.C., mercury cup contacts. Rt, relay which served to hold intact the lower chronoscope magnet circuit until the shutter descended and made contact at M.C. D. The electrical set-up for presenting sounds to groups. Tu., a 112A radio tube. Ph., phone. A, audio oscillator. C, condenser of 0.1 m.f.d. capacity. n and rt, resistances of 250,000 ohms each. P, potentiometer by means of which the loudness of the sound was regulated. E. Set-up for timing the interval between the make and the break of contact in the time control relay, Uj. //, Hipp chronoscope. Rt, relay which effected a break in the upper magnet circuit of the Hipp. Ri, shunt relay which served to hold intact the lower magnet circuit until Ri was activated. This relay had to be closed by hand before each time measurement was made.