Hubs are the center of the outside part of the yoyo. On a yo-yo with a long axle, the hubs often include a nipple, spike, or stub, which contains the axle threading inside. On yo-yos with shorter axles, the hub can be totally flat, allowing for horizontal fingerspin tricks.
Some yo-yos with longer axles have been designed to accept hubstacks, a plastic shell containing a ball bearing which is mounted onto the axle stub. These hubstacks can be held with the fingers, enabling the yo-yoer to directly adjust the plane of the yo-yo while spinning, as well as performing hubstack based tricks.