Q: I’ve seen the terms rhythmic and isometric in the hymnal. What does that mean?
A: Now, admittedly, although I, Ambrose, was a composer of numerous hymns, the terms “rhythmic” and “isometric” were, at first, unfamiliar to me. I am most acquainted with a certain style of plainsong hymnody. Nevertheless, I have done some research, and I have an answer for you!
There are two main styles of rhythmic movement in hymns. One consists of the sturdy rhythms with regular patterns of mostly quarter notes and a few longer note values, especially for phrase endings. This style is sometimes called isometric” (“same meter”); it was common in the 18th century and is now associated with the chorale harmonizations of J.S. Bach. Most of the older tunes found in American hymnals are isometric. The other style, known as “rhythmic,” consists of more irregular and often syncopated rhythms, such as those often found in the Reformation-era chorales (c. 1520—1550), which were originally sung without accompaniment. In The Hymnal 1982 you see these two rhythms illustrated in “Now thank we all our God” (Hymns 396/7), “A mighty fortress is our God” (Hymns 387/88), and “Sleepers wake!” (Hymns 61/2).
Thank you for posing this question, because it has expanded my own exposure to how much hymnody has evolved since the fourth century!