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.
Epee does not use the right-of-way in keeping with its dueling origin - he who first gains the touch earns the point. Or, if both fencers hit within 1/25th of a second of each other, both earn a point. However, it is equally important to have a sound defense for epee, since the entire body must be protected from a touch.