I've been spending a lot of time in the hymnal today working on Advent music for my church. One of the mysteries of my childhood (like why are there sometimes extra digits after the zipcode on mail, or why do composers sometimes use an f double-sharp instead of writing g natural), was what the dotted numbers at the bottom of the hymnal mean. If you've never looked it up, it's a guide to the metrical feet in each hymn.
For example, Hyfrydol (which sounds like a skin rash that you can only contract in Cardiff) is 8.7.8.7.
(1)come (2)thou (3)long (4)ex(5)pec(6)ted(7)je(8)sus
(1)born (2)to (3)set (4)thy (5)peo (6)ple (7)free
(1)from (2)our (3)fears (4) and (5)sins (6)re (7)lease (8)us
(1)let (2)us (3)find (4)our (5)rest (6)in (7) thee
Once I figured this out, I naturally wanted to pull all the old hymns apart and put them together again in funny ways. So, once you know that Hyfrydol is 7.8.7.8, you realize that you can sing the text of Come, Thou Long Expected Jesus to the tune of Abbot's Leigh (which sounds like a venereal disease you can only contract in a monastery), which usually has the text "Glorious Things of Thee Are Spoken).
Similarly, the hymn tune Aberystwyth (which looks like the sort of thing that a toddler would text to your boss when they walk off with your phone) is 7.7.7.7, so the words "Watchman, Tell Us of the Night" can be sung to the tune we usually associate with "Come, Ye Thankful People Come."
Away in a Manger 11.11.11.11 already has two common settings, but it can be sung to the tune of Immortal, Invisible. (St. Denio, the patron saint of gambling in Atlantic City and wearing size XXL polo shirts)
Angels We Have Heard on High can be sung to "Jesus Loves Me." (Hymn tune name: "Jesus Loves Me")
Joy to the World can almost, but not quite, be sung to Amazing Grace. If I were to every to program it that way I'd probably have it sung to the Amazing Grace tune up until heaven and nature start singing, and then switch to the old tune.
It Came Upon a Midnight Clear (Hymn Tune Carol, which sounds like a secretary from HR who is always in a bad mood) can be sung to I Sing the Mighty Power of God (Ellacombe, which sounds like part of a balanced breakfast.)
Once In Royal David's City (Irby, which sounds like a place where you get a roast beef sandwich in Minnesota) can be sung to Open Now the Gates of Beauty
Angels From the Realms of Glory can be sung to the tune of Let All Mortal Flesh Keep Silence.
Fun bonus list of funny hymn tune names:
Wedlock
Crashing Waters
El Camino
Feed Us (This is the official hymn tune of our boys)
Hankey
Jonathan's Tune
Laying Down (This is the official hymn tune of the parents)
Mystic Bamboo Song
Redhead 76