Interchangeable needles are circular needles with detachable tips.
Simply pick the needle size and cord length you want, fix them together and off you go!
They often come in sets containing a variety of needle sizes and lengths of cord, or you can buy the components separately.
Circular needles can be used flat or in the round, for everything from small-circumference projects like hats and socks, to larger projects like blankets, where they help to accommodate the stitches and distribute the weight.
You can find cables, cords and connectors to fit the interchangeable needle tips HERE.