The Down Judge is one of a few officials that officiates a football game. The down judge is positioned near one sideline, at the line of scrimmage, working together with the line judge, who is on the opposite sideline, in the same position. The down judge has to look out for different kinds of faults as a play starts and develops, with a focus on the area around the line of scrimmage and a few yards ahead, and the nearest sideline.
Before the snap, the down judge, in conjunction with the line judge, will keep an eye open to make sure no scrimmage infractions such as illegal motions, offsides, false starts, and encroachment; all very commonly committed by players; are committed.
After the snap, it's the down judge's job to be attentive to the nearest sideline, and rule if a player was out of bounds when receiving a pass or running with the ball. Some plays near the sidelines can be very polemic and completely change a football game, therefore, the down judge's alertness to the sideline is crucial for a game to run smoothly. When a play ends, the line judge comes in action to check how many yards a team advanced, and signal it to the other officials. The down judge is also the official in charge of the chain crew, and when necessary he comes into the field to measure a first down.
In 2017 the NFL renamed the position from head linesman to down judge. The change was made after the league found necessary to have a gender neutral name, after Sarah Thomas was hired as the NFL'S first full-time female judge.