From Hymnary Wiki
A hymn number is the number of a hymn in a hymnal.
- Each hymn number in a hymnal must be unique.
- Each hymn number needs to be accounted for, even if it is not a hymn, such as a responsive reading, prayer, etc. If it is not a hymn, put an explanation of what it is in the Notes field.
- If there is more than one musical thing under one number, such as multiple song responses or alternate harmonizations, add an "a", "b", etc. to the number, if necessary, such as 72a, 72b. It is preferable to try to fit the information under one listing if possible; for example, more than one composer can be listed but not more than one tune name or meter.