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Crew that manages signal poles in gridiron football

In gridiron football, the chain crew (commonly known as the "chain gang") is a crew that manages signal poles on one of the sidelines. There are three primary signal poles: the "rear rod" that marks the beginning of the current set of downs, the "forward rod" that marks the line to gain, and the "box" that marks the line of scrimmage. The two rods are attached at the bottom by a chain exactly ten yards long, while the "box" displays the current down number. The chain crew, under the direction of the head linesman/down judge, signals the officials' decisions; it does not make decisions. Players look to the chain crew to see the line of scrimmage, the down number, and the line to gain. Officials may rely on the chain crew after a play (incomplete pass or penalty) whose outcome depends on the original spot of the ball. This also includes bringing the chains onto the field whenever an accurate measurement is needed to determine if a first down has been made.

Three members of the chain crew hold poles upright. The bottom of each pole is placed along a sideline to denote a line across the gridiron to the opposite sideline. A "rod man" holds the "rear rod" that marks the beginning of the current set of downs. Another "rod man" holds the "forward rod" ten yards toward the defense's goal line from the rear rod. This marker indicates the line to gain, which the offense must reach in their series of four downs in order to retain possession of the ball. The two rods (sometimes known as "sticks") are attached at the bottom by a chain exactly ten yards long. The chain is always taut so that the rods are ten yards apart. The "box man" holds a pole that marks the line of scrimmage. There may be additional chain crew members. A "clip man" is discussed below. In the NFL, additional chain crew members have additional tasks, such as to relieve the line judge of the clerical task of recording all assessed penalties. The chain crew does not wear protective gear as players do. A routine instruction by officials to the chain crew is to withdraw or drop their signals, and move back, if the play comes toward them so as to endanger them. Often, the signals use bright orange color, are padded, and have break away components for safety.

For games at all levels except the NFL, the chain crew operates on the side of the field opposite the press box (usually the visiting team's sideline). In the NFL, the chain crew switches sides at halftime; the referee determines their initial placement. In the NCAA, the chains are located opposite the press box for the entire game. In the first half, the line judge is located opposite the press box and supervises the chain crew, with the head linesman on the press box side of the field. In the second half, the head linesman and line judge swap sides of the field like in the NFL, with the linesman supervising the chain crew. This practice started in 2014. In the NFL and other venues where there is a zone behind the sidelines, all three poles are placed somewhere along the back line of this zone. Otherwise, the poles are placed along the sideline.

For professional and college football games, an auxiliary chain crew operates on the opposite side of the field, supervised by the line judge. Their presence lets players and officials look to either side of the field for information. The auxiliary chain crew also includes a member holding the drive start indicator, which is placed at the beginning of a team's drive and stays there until the team loses possession. This indicator is only used for statistical purposes to calculate the distance of each drive. It looks similar to a "stick" and has an arrow (or occasionally a large "X") that points in the direction the offensive team is going. The NFL eliminated the drive start indicator in 2018. A device on the clip indicates which numbered line this is. The clip and the device let the chain crew restore the position of the rods after a mishap. In leagues such as the NFL with Instant Replay, there may be multiple clips to let the rods be repositioned after a play is reviewed and reversed. On plays where there is no line to gain (a series of downs that starts within 10 yards of the goal line, a try after touchdown, or a kickoff), the rod men lower or lay down their signals, but the box man continues to mark the line of scrimmage.

The chains are brought onto the field whenever the referee needs an accurate measurement to determine if a first down has been made. A team may also request an accurate measurement to determine how far they have to reach for the first down.

Before the game, many linesmen attach a piece of tape at the midpoint of the chain. The linesman can compare the line of scrimmage to this mark at the start of a play and know whether a 5 yard penalty against the defense will result in a first down. If so, the linesman's typical hand signal to the line judge across the field is to extend the arms with the thumbs pointing toward one another. Such a gesture with thumbs pointing away signals that there are more than 5 yards to gain.

All levels of organized football have used the chain crew. The "box" (down marker) has evolved over time. In the pre television era, it was a rod (shorter than the height of the operator) with a triangular pointer that rotated to show the official the down number. It was replaced with a much larger, two sided flipper system when television became widespread; flipper style down markers are still in use in scholastic and amateur football. A more modern model is the Dial a Down, in which compact levers display a new down number over the entire face of the "box"; this model is used at most college and professional games.

Early attempts and proposals to use precision automatic chain instruments were hindered both by costs and potential hazards (early proposals required metal rails; a 1990s system used visible lasers). For the 1974 season, the World Football League used the "Dicker rod," a proprietary stick approximately in length, which allowed measurements to be made with one person instead of three. Lazser Down, which uses wideband radio waves to precisely spot and measure distances, has been used in college football, the Alliance of American Football and the XFL. When the USFL and XFL merged to form the United Football League, they continued to use a similar technology, "TrU Line."