A painstaking determination of pure-tone threshold versus frequency in normal ears using a headphone reveals a remarkably consistent microstructure of hills and valleys. A large number of determinations of the frequency at the peak of each of the threshold hills reveals a standard deviation of less than 10 Hz for stimulus frequencies from 200 to 5000 Hz. The widths of the intervening valleys correspond remarkably closely to commonly accepted critical bandwidths for the stimulus frequencies concerned. Must surprisingly of all, the perceived pitch of the stimulus jumps quantally from one valley to the next at intensities very close to threshold and remains constant as stimulus frequency is varied within the “passband” of the valley. If intensity is increased just slightly above the top of a hill at the hill frequency, a mixture of the pitches including interactions is heard. If intensity is increased a few decibels still further, an apparently smooth variation of a single pitch is observed as stimulus frequency is varied from side to side of the hill frequency. Hill frequencies are in general different for the two ears. Intensity difference from top of a hill to the bottoms of the adjacent valleys is typically 12 dB. These findings strongly support the place theory of hearing for pure-tone stimuli.