In most primates the wrist is a stiffer arrangement of small bones. In gibbons the joint between the forearm and the hand allows a wide arc of rotation, letting the hanging body swing through and change direction while the grip stays fixed. The hand itself works as a hook: the fingers are long and curved, and the thumb is set far back near the wrist so it does not obstruct the hook.
The arms are much longer than the legs, giving an intermembral index well above one hundred and twenty, the highest of any ape. Combined with a short, mobile trunk and no tail, this produces a pendulum tuned for a long, energy-cheap swing.
The common picture of continuous flight through the canopy is overdrawn. Gibbons also walk bipedally along branches with arms raised, and leaping and climbing make up a substantial part of ordinary daily travel.