The most common type of Thong is the Simple thong....

This thong is just like any other bikini in the front, but the back is a small strip of material that runs across the tops of the butt and a upside down triangle has an even smaller piece of material at it's lower apex that runs down the crack of your gluts and connects to the underside of the bottom of the front piece.  Although this is the most common, I find it to be the least comfortable.  The more material that you have in your preverbial butt crack the more uncomfortable it is.

Here are a few common thongs that I have modeled.