A down is the period of action that begins when the ball is put in play and ends when the ball is declared dead. Most downs start with a snap from scrimmage, but kickoffs and safety kicks start when the ball is struck. An offense has four downs or fewer to advance the 10 yards required to gain a first down, which allows them to maintain possession and earns them another four downs. The initial down in a series of downs is called a first down. Subsequent downs are numbered sequentially — second, third, or fourth.