Number 117 was my favourite as a child. Well, it has only two verses. It's also cheerful and snappy, and you can understand what it's going on about:
O praise the Lord, all ye nations: praise him, all ye people.
For his merciful kindness is great towards us: and the truth of the Lord endureth for ever. Praise ye the Lord.
The word psalm has given us various other English words. A psalmist writes psalms (the Psalmist, with a capital P, is King David); psalmody is to do with singing psalms, or setting them to music; a psalter is a book with the psalms in it.
And then it's not much of a stretch to get to a psaltery, which is a sort of stringed instrument:
This very fed-up psaltery player comes from an illustration in the Gorleston psalter.
But what about a psalterium, which is...
Yes, you're quite right. Of course: you've guessed it.
A psalterium is the third stomach of a cow.
Well, because of this:
Word To Use Today: one beginning with psal-. The song word comes from the Latin psalmus, from the Greek psalmos, song accompanied by a harp, from psallein, to play the harp. (This explains psaltery nicely, doesn't it.)
Psalterium comes from the Latin psaltērium, which means psalter, because a psalterium has many folds that look a bit like the pages of a book.
Here's a model of one which completely fails to illustrate this:
Model of the psalterium and abomasum of a cow. Photo by Wagner Souza e Silva, Museum of Veterinary Anatomy FMVZ USP The psalterium is the spiral bit.
Still, this photo of one of the leaflets inside the psalterium helps just a little: