UPC codes

This seems like a question that someone here could answer. While trapped in a tent for a day, I figured out how UPC symbols were coded. However, all symbols have a digit to the right, two groups of digits under the code, and a digit to the left. I notice that in almost all cases (I have only found one exception so far), the number on the right is smaller than the number on the left.

What are those two numbers. I feel that they have something to do with error checking (parity sums or something like that), but I can't figure out how they work.

http://www.barcode-graphics.com/info...upccontent.htm

Cool, the fact that the left number was almost always higher than the right number was coincidental. I spent hours puzzling over that! On the other hand, it was a cold and rainy day, so I wasn't into moving about very much.