when most people were not musically literate and learned melodies by rote, it was a common practice to sing a new text to a hymn tune the singers already knew which had a suitable meter and character.

There are many hymn tunes which might fit a particular hymn: a hymn in Long Metre might be sung to any hymn tune in Long Metre, but the tunes might be as different as those tunes that have been used for centuries with hymns