What's the purpose of the half-moon thing around the sound hole of a guitar?
Dear Straight Dope:
Why does a guitar usually have that little half moon thingie on one side of the hole?
I presume you're talking about the plastic (or tortoise-shell) object that is attached to the body of the guitar down-stroke from the sound hole.
It's a pick guard. It's designed to prevent damage to the actual finish of the guitar by overly aggressive strumming. It also offers the guitarist a place to rest his/her hand when performing certain intricate picking without getting skin oil on the finish of the guitar.
Since the pick guard is replaceable without having to refinish the wood, this is considered a good thing for those who are actually concerned about the appearance of their axe.