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. On-field measurements 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. 5-yard mark 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. History All levels of organized football use the chain crew. One notable exception at the professional level was the World Football League of 1974 to 1975, which used the "Dicker-rod," a proprietary, 90-inch stick that allowed measurements to be made with one person instead of three. The Dicker-rod was never used outside the WFL. 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. Electronic down markers using LEDs are emerging as the next advance. However, further application of technology to measure the lengthwise position of the football, such as sensors inside the football, may be resisted based on affection for the human element. 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