An analysis of a sample of polyphonic keyboard works by J. S. Bach shows that the prevalence of different vertical intervals is directly correlated with their degree of tonal consonance. A major exception to this pattern arises with respect to those intervals that contribute to tonal fusion. The prevalence of the latter intervals is negatively correlated with the degree to which each interval promotes tonal fusion. Bach's avoidance of tonally fused intervals is consistent with the objective of maintaining the perceptual independence of the contrapuntal voices. In summary, two factors appear to account for much of Bach's choice of vertical intervals: the pursuit of tonal consonance and the avoidance of tonal fusion.