A stepless gearbox lets a machine run from a standstill to maximum speed with no distinct ratio changes or ladder-rung gears.
Two sorts of CVT have been around long enough to grow whiskers. The simplest uses a pair of variable-diameter pulleys or sheaves connected by a belt; you’ll know them from combines, some ATVs and mopeds.
Hydrostatic drives uses oil lines to connect a variable-displacement pump to a fixed- or variable-displacement motor. Moving a control lever sends more or less oil to the motor, changing its speed.
Both systems deliver stepless speed change. Both are limited either by torque capacity (belt drive) or relatively low efficiency (hydrostatics).
Modern tractor CVTs blend the high efficiency of a mechanical drive with the speed flexibility of hydrostatics. Two basic designs currently rule: