A scrimmage down, or down, is just a fancy word for a play starting at the line of scrimmage.
When the offense gets the ball they are given a set of four downs to move the ball on the football field. These scrimmage downs are called:
The scrimmage line shows the spot of the ball on the field. It's where all downs begin and end.
Downs always begin with a snap or hike of the ball from the center to the quarterback. Both are players on the offense. A snap or hike is a backward pass throw the legs.
After the snap, the ball is live and the down begins.
The down ends when the ball is declared dead by officials. The down ends for a variety of reasons:
The chain crew will mark the new line of scrimmage, first down line, and the distance between the two lines after every down. The chain crew is a group of referees that stand on the sidelines.