Yesterday in church we sang a hymn to the tune Kingsfold; the hymn was "I Heard the Voice of Jesus Say". I'd never actually heard the hymn before, although it's not an obscure one, but I recognized the tune immediately, because it's one of the tunes that can be used for the nineteenth-century Irish ballad, "Star of the County Down". And thus it reminded me of the close link between true hymnody and folksong, both of which traditionally have made sharp distinctions between text and tune, and thence have derived both a subtle variety and considerable ingenuity.
"I Heard the Voice of Jesus Say" is a good example of this, actually, because while I heard it to Kingsfold, it's actually more often sung to the tune Vox Dilecti, and they are very, very different tunes. You can get a sample of Vox Dilecti here, and a sample of Kingsfold here. What both tunes share is an 220.127.116.11 hymn meter, which is also known as Common Meter; any text that can be sung to any 18.104.22.168 tune can be sung to any other 22.214.171.124 tune, and there are lots and lots of them. And it's a bit more expansive than that. You can, by lengthening or shortening syllables (on which hymn meter is based), sing texts to tunes with different meters.
For instance, "O Little Town of Bethelehem" is usually sung to the tune St. Louis, which is a 126.96.36.199 tune; the text and the tune are so linked that almost nothing else is sung to that tune. And it's a good pairing; St. Louis has a relatively quiet and reflective feel, especially if you sing it slowly. However, you occasionally find people singing "O Little Town of Bethlehem" to the slightly more upbeat Forest Green. You can find a sample of St. Louis here, and a sample of Forest Green here. But you could also sing it to Kingsfold, which is also 188.8.131.52. It would sound strange because we're not used to that text sounding jaunty, and Kingsfold sounds somewhat jaunty, as we find when we listen to someone singing "Star of the County Down". Likewise, you could sing it to Vox Dilecti. But you could also take a 184.108.40.206 tune, like King's Lynn, and sing it to that. I think that text and tune would go very well together. You could sing it to Llangloffan, which is another 220.127.116.11 tune, and it would sound different again.
Perhaps the best-known hymn tune in the world is a slightly modified New Britain, also an 18.104.22.168, because the text usually sung to it is "Amazing Grace". But you could sing "Amazing Grace" to any 22.214.171.124 tune. Some of them would sound very strange, just as singing "O Little Town of Bethlehem" to New Britain would sound strange because the accents and pauses would be strange for the text. But you could do it. And, of course, this goes much farther than the standard folk tunes; one of the things Baptist youth groups often do is sing "Amazing Grace" to the tune of "Theme from Gilligan's Island" or the tune of "House of the Rising Sun". For that matter, you could reverse it. Or sing "O Little Town of Bethelehem" to the tune from Gilligan's Island; talk about jaunty.
Of course, what this means is that there are no unsingable hymns. If you don't like the tune, you can pick a new tune. And if you don't know the tune for a lyric, you can still pick up the text and sing it right off if you know the hymn meter (which is just the syllable count of the lines) and a corresponding tune. It's a system designed for a people who sang for fun, for heartfelt expression, for whiling away the time, for a people whose approach to music was not passive but active. Everyone was a singer; some did it better than others, but the tunes were common property. Genuine folk music, music of the folk. People tend not to disconnect text and tune in this way today, and I think it's for two reasons. First, we vastly overrate originality, and so every text has to have its special tune and vice versa, unless we are doing parodies, which (1) is silly and (2) makes singing the special province of those who control the tunes and the texts. Second, we are not a singing people but a listening people, timidly letting others sing because their voices are better. This also (1) is silly and (2) makes singing the special province of those who sing a wide range of tunes well. In fact everyone can sing some tunes well, and nobody needs to be original to have great songs. And singing is what human beings do to express their hearts, and as such it is something suitable to everyone.