Singer's Prewound Class 15 Bobbins are easy to use and provide the utmost convenience. This set includes 6 black and 6 white bobbins, threaded with 50 yards of high-quality, 100% spun polyester thread. The 12 Prewound Class 15 Bobbins come packaged in a handy plastic case that can be reused. Please verify your machine's bobbin size in the manual to ensure this set will work for you. Remember, never wind new thread onto a partially filled bobbin, as you risk breaking or tangling thread.