The rugby union position traditionally numbered 15 is fullback.
The fullback is the last line of defence behind the other backs. This player is expected to field kicks, communicate defensive positioning, cover open space, and return the ball from deep. Fullbacks also join attacking moves when they see an opportunity.
Traditional shirt numbering assigns numbers 1 through 15 to the starting team. The fullback wears 15, while the two wings wear 11 and 14, the centres wear 12 and 13, and the half-backs wear 9 and 10.
Modern rugby allows tactical substitutions and positional flexibility, so a player wearing a different number may perform fullback-like duties. The traditional number remains 15, however, and is a standard reference point for team selection.