The ELVIS project had three locally based teams (in Canada, at McGill, in Scotland, at Aberdeen, and in New England, USA, divided between MIT, the lead, and Yale), each of which focused on a different aspect of the overall research program: using computers to understand musical style. The central unifying concept of the ELVIS project was to study counterpoint: the way combinations of voices in polyphonic music (e.g. the soprano and bass voices in a hymn, or the viola and cello in a string quartet, as well as combinations of more than two voices) interact: i.e. what are the permissible vertical intervals (notes from two voices sounding at the same time) for a particular period, genre, or style. These vertical intervals, connected by melodic motions in individual voices, constitute Vertical Interval Successions. In more modern terms, this could be described as harmonic progressions of chords, but what made ELVIS particularly flexible was its ability to bridge the gap to earlier, contrapuntally-conceived music by using the diad (a two-note combination) rather than the triad (a combination of three notes in particular arrangements) as a basis (since triads and beyond may be expressed as sums of diads)
