The first phase of Cantus Chant ID Numbers was based in the assignments given by René-Jean Hesbert in Corpus Antiphonalium Officii (Rome, 1963-79). These volumes, which index and provide the full texts of the chants found in twelve of the earliest Office antiphoners, include 4-digit numbers with letter-suffixes for some genres. Chants occurring in Cantus inventories that were not included by Hesbert in CAO were assigned arbitrary numbers within each manuscript index.
In a later development of this numbering system, two digits were added:
- "00" precedes all CAO numbers, thus preserving Hesbert’s assignments but allowing for future growth of the database
- six digit numbers (plus applicable letter-suffixes) with numerical prefixes in accordance with Hesbert’s scheme ("10" = invitatories, "20" = antiphons, etc.) were assigned to all chants not included in CAO, in order that the chants could be cross-referenced among all sources in Cantus
The third and current phase of numbering began after the implementation of Cantus Index, where users can enter new texts online for automatically-generated numerical assignments. The prefix designations "a" and "g" were introduced; these identify (for the most part) Office chants ("a") and Mass chants ("g").
The six-number format and the “g#####” or “a#####” (plus suffixes) now identify each chant in Cantus Index and Cantus Database.
Older designations of "CAO 1234" or "cao 1234" or "CANTUS ID 001234" will now merge into a uniform format of "Cantus 001234".
Over three decades, Cantus has created tens of thousands of new identification numbers for individual chant texts. The Cantus numbers both incorporate and augment the system begun by Hesbert in CAO.