Although both codes are played on similar sized rectangular fields, the dimensions of rugby union fields can vary up to maximum size that is larger than the fixed size of American football fields. Rugby union fields are limited to a maximum length of 144 m long (and 100 m between goal lines) and width of 70 m, while American football fields have a fixed length of 120 yd (110 m) long and width of 160 ft (48.8 m). Both sets of measurements include scoring zones at each end. These are fixed to 10 yd and called the end zone in American football, but of unspecified length in rugby union.  Lines
The border between the regular field of play and a scoring zone is called the goal line in American football and the try line in rugby union. The outer perimeters of both fields are demarcated with sidelines (ends of rugby union field border the scoring zone being called dead ball lines and longitudinal sides touch lines). The central playing field of rugby is divided into halves by a halfway line; however, when the American football field was shortened, the centre line was replaced by the 50 yard line which is simply referred to as the 50 yard line or midfield. Additional lines differ markedly with American football fields marked at every 5 yard interval, whereas rugby union fields only have two further solid lines called the 22 metre lines and four broken lines each halving a half (resulting in four quarters and being translated as "quarter lines" in some languages). The dotted lines are made of two 10 metre lines on each side of the halfway line and two 5 metre lines before each goal line. Rugby union fields also have another set of dotted 5 metre lines. The yard lines in American football are vitally important during game play, because a team's advance is measured against these lines, which in turn determines ball possession, whereas the halfway, 22 and 10 yards lines only determine the position of players during various rugby union kick-offs (which could be from the halfway line or 22s) or line-out in the case of the broken line 5 yd from the side lines.  Goalposts
Both codes also have goalposts at each end of the field: on the goal line in the case of rugby union, but further back in American football on the back of the end zone (in Canadian Football at the goal line). American football goalposts consist of two vertical posts 18.5 feet apart (24 feet in high school football) rising from a horizontal crossbar mounted on single (usually) post raising it 10 ft off the ground (resulting in a combined Y-shape of sorts). Rugby union goalposts are 5.6 m (18.3 ft) apart and extend vertically from the ground being connected by a crossbar at 3 m (9.8 ft), creating an H-shape. In both cases, only kicks passing between the uprights and above the crossbar score points. The scoring areas of both types of goalposts are therefore similar.  Advancing the ball
In American football, the team that is in possession of the ball, the offense, has four downs to advance the ball 10 yards towards the opponent's end zone. If the offense gains 10 yards, it gets another set of four downs. If the offense fails to gain 10 yards after 4 downs, it loses possession of the ball. The ball is put into play by a snap. All players line up facing each other at the line of scrimmage. One offensive player, the center, then passes (or "snaps") the ball between his legs to a teammate, usually the quarterback. Players can then advance the ball in two ways:
•By running with the ball, also known as rushing. One ball-carrier can hand the ball to another; this is known as a handoff. A ball-carrier can also perform a lateral or backward pass as in Rugby. •By passing the ball forwards to a team-mate as long as the passer is behind the line of scrimmage. A down ends, and the ball becomes dead, after any of the following: •The player with the ball is tackled.
•A forward pass goes...