There are 3 different sizes of machine embroidery bobbins that we sell, style L, M, and A (also know as Class 15). These do not stand for large and medium. The L & Class 15 embroidery bobbins are the smaller bobbins, they are about the size of a nickel. L fits most commercial embroidery machines and many home machines, A, or Class 15 fit many home machines only. Style M machine embroidery bobbins are about the size of a quarter. They are also sometimes called jumbo bobbins. Your embroidery machine must have been manufactured to take the M bobbin, or they will not fit.