The CheckerBoard

Since my only football playing experience is as a 90 lb bench warming wide receiver in middle school, my knowledge of the game's intricacies is fairly limited. Question for Wes, Ryan, or any of you former football players out there...

I've heard several references this season to the Vols offensive line "grading out" pretty well after games, even in losses. Wes also had a couple tweets about this tonight. What does that mean? How does an offensive line get graded? Is it all quantifiable statistics or is it a coach's subjective judgment? Is it an individual or a team grade? What is a "good" grade (from Wes's tweet, it seems that 80+ is good)? How do the Vols "grades" compare to other teams or is that something you can even compare?

Thanks, guys.