It's A Psalm Not a Psalms

I know this is a little nit picky, but it's a personal pet peeve of mine.  Ray Lewis of the Baltimore Ravens was wearing this shirt last Sunday when conducting some of his post game interviews.

From LA Times

In case you can't read it it says "Psalms 91."  The entire group of them are called psalms, just as a group of the balls used for his sport are called footballs, but when it is referring to just one then we remove the "s" to make it singular.  He does not catch a footballs plural, he catches a football  singular (or not as the case may be from his last game), and it is a psalm singular when referring to only one, not a psalm

So let's reiterate one more time.  When you are talking about multiples or about the whole book it is Psalms, but when you are only talking about one it is Psalm.  Singular not plural.