Polyester Bobbins

Factory wound prewound bobbins hold up to three times more thread than self-wound bobbins.

What does Class 15, L-style and M-style mean?

Class 15, L-style & M-style are the three most common sizes of prewound bobbins. Some longarm machines (A-1, Gammill, and Handi Quilter) take the M-style bobbin, which is larger in diameter than both L-style and Class bobbins. 

L-style bobbins and Class 15 bobbins are nearly identical in diameter (Class 15 bobbins are about 0.5 millimeters larger in diameter).