Musicologists and philosophers have commonly attributed distinctive qualities to individual musical pitches, and absolute pitch (AP) possessors recognize and recall notes and keys with immediacy and accuracy, leaving little doubt that they are aware of such characteristics. Bachem proposed that these distinct tonal qualities underlie the rapid and accurate judgments that he identified as genuine AP, and he defined the qualities as tone chroma (TC). The TCs of notes and keys, and of notes separated by a musical octave ( pitch class) are frequently expressed in visual terms, and studies of synaesthesia, the association of intersensory stimuli, provide clues to the systematic variation of TC qualities. The historical literature relating to note and key characteristics is commonly overlooked in the study of AP, however, and the article seeks to address this problem. Long-standing conclusions are reviewed, leading to the hypothesis that TC sensitivity can derive from an awareness of variations in the acoustical beats that occur in the tuning of instruments to equal temperament and which are perceived with particular clarity in organ tuning. This acoustical hypothesis is supported by modern neuroscientific findings and was predicted by theoretical observations in the literature as long ago as three centuries. In a follow-up article, Thurlow and Baggaley discuss the role of synaesthesia-type judgments in musical skills not previously regarded as absolute: for example, a perfect touch capacity observed in keyboard players.

The previous article in this series reviewed the historical and modern academic literature concerning the distinctive characteristics of individual musical notes and keys. It stressed Bachem’s definition of tone chroma (TC) as the quality that allows notes/keys to be identified instantly and accurately by musicians possessing the type of absolute pitch (AP) that Bachem described as genuine. TC qualities were shown to vary in the same systematic manner as the second-order acoustical beats predicted to accumulate during the tuning of instruments to equal temperament. This article offers further evidence for the connection between TC and acoustical sensitivity, as indicated by an examination of paracusis musicalis (PM), the shifting of the pitch sense with age to a level sharper or flatter than its original level. It is also noted that AP judgments have been shown to be based on kinaesthetic and tactile sensations, which perform the same cueing functions as auditory TC, and that types of AP judgment may, therefore, exist not typically identified as absolute: for example, an absolute or perfect touch capacity observed in keyboard players. Evidence of this capacity supports the theory of instrument-specific AP.