What counts as knocking a fence down in show jumping?

• A knock-down: "the whole or any upper part of the same vertical plane of it falls, even if the part that falls is stopped in its fall by any other part of the obstacle".
• Or: "at least one of its ends no longer rests on any part of its support".
• The penalty: four penalties under Table A.
• Only the top: where parts are stacked in the same vertical plane, only the fall of the top part is penalised.
• A spread: the fall of one or several top parts counts as one fault.
• Touches: touches and displacements short of a fall are not penalised. In doubt, the ground jury decides in the rider's favour.
• Flags and filling: knocking down a flag, or the hedge or other filling of an obstacle, is not a knock-down.
• Badly built: no penalty if the obstacle was improperly built and was jumped correctly.
• The last fence: if its top element falls after the finish line but before the rider leaves the arena or the next bell, it counts.
• During a refusal: an obstacle knocked down by a horse that stops is penalised as a refusal only, with six seconds added while it is rebuilt.
• In the way: if a fallen part would obstruct a later obstacle, the bell is rung and the clock stopped while it is cleared.

The glossary has the knock-down, the rub, the fault, the obstacle and the clear round.

The cups are shallow so that a pole falls before a horse does. The rule then has to say exactly when a fence is "down", because poles bounce, roll and hang. Penalising only the top element of a plane keeps one mistake at one fault.