There is the class 66 bobbins which is for a drop in type machine then there are the touch and sew etc with drop in bobbins on the newer machines. A lot of times a class15 bobbin is used in the machines with a bobbin case whether Singer 15 or others. Others with a bobbin case: the 301 and the FW use it's own type. The 306 etc use an L bobbin - There are long bobbins but that doesn't count in this case. This is not totally right so be sure to look up what you need in your owner's manual. Manuals can be found on line.