Two tones of different frequencies alternated successively five times per second. When the difference in frequency was small, the alternation sounded like a continuous up‐and‐down movement of the pitch. When the difference in frequency was large, the alternation sounded like two unrelated, interrupted tones. The transition point between these two perceptual organizations is called the trill threshold. The trill threshold was measured as a function of frequency for 14 subjects; the results are summarized in Fig. 1.