Slide them on your needles to lock stitches securely in place between knitting sessions. 

Use them as end stops and remove your interchangeable needle tips for another project.

For larger needles, remove the needle tips first, then add your cable lock to lock your stitches in place.

Small - Fits needles up to US 4

Medium - Fits needles up to US 8

Large - Fits needles up to US 10.5