1st:  Optimal glass with little to no imperfections.

B+:  Close to first quality, may have a slight imperfection like a small inclusion or minor air content.

B:  Can have multiple imperfections consisting of minor inclusions, air content, streaking, and/or variations in diameter.

2nds: May have inclusions, air content including long bubbles, streaking and minor wall weight issues. Heavily off wall weight tubing is generally listed in a separate category.