The axle is what keeps both halves of the yo-yo together. Originally the yo-yo would spin freely on the axle in the string loop. After the invention of the transaxle sleeve and the ball bearing, the transaxle or bearing sits on the axle to reduce friction, making for longer spin times to allow for more complex tricks. Some companies produce yo-yos that feature interchangeable axle components, such as Henrys' AXYS and One Drop's Side Effects.