Class 66 Plastic Bobbins - Curved sides. Fits some of the sewing domestic/home machines like Brother, Baby Lock, Singer.
Class 15 - Most common style - Fits most Japanese machines: Brother, Riccar, Singer, Janome, Kenmore, White, and Baby Lock.
(Will not fit Pfaff, Janome, Elna, Bernina or Husqvarna)