One of the most difficult concepts to visualize in foil and sabre fencing is the rule of right-of-way. This rule was established to eliminate apparently simultaneous attacks by two fencers.
In essence, right-of-way is the differentiation of offense and defense, made by the referee. The difference is important only when both the red and green lights go on at the same time in foil and sabre. When this happens, the winner of the point is the one who the referee determined was on offense at the time the lights went on.
The Sabre is the modern version of the slashing cavalry sword, and is similar in length and weight to the foil. The major difference is that the Sabre is a thrusting weapon as well as a cutting weapon (use of the blade). The target area is from the bend of the hips (both front and back), to the top of the head, simulating the cavalry rider on a horse. The Sabre fencer's uniform includes a metallic jacket (lamé), which covers the target area to register a valid touch on the scoring machine. The mask is different from foil and Epée, with a metallic covering since the head is valid target area.
Just as in foil, there are two scoring lights on the machine. One shows a green light when a fencer is hit, and one shows a red light when the opponent has hit. Off-target hits do not register on the machine. Sabre also follows the rules of "right of way".
Arizona State Fencing Club