American football positions have slowly evolved over the history of the sport. From its origins in early rugby football to the modern game, the names and roles of various positions have changed greatly, some positions no longer exist, and others have been created to fill new roles.
Origins in rugby
Being variants of 19th century rugby football,Football: The Rugby Union Game, 1892, Frank Marshall, ed., pp.18–22 American and Canadian football position nomenclature has its origin there. Early rugby did no more than distinguish in tactics between the great bulk of the players who played as forwards, and the relative few who played back defensively as "tends", as in goaltenders. After a while, the attacking or at least counterattacking possibilities of playing close behind the scrimmage (which later came to be called "scrummage") came to be recognized, and some players stationed themselves between the forwards and tend as "half-tends". It was seen that the players outside scrimmage (the "pack", i.e. the forwards) were not limited to a defensive role, the tends and half-tends were renamed "back" and "half back" positions.
As the game advanced, backs positioned at different depths (i.e. distances behind the forwards) were further differentiated into separate positions. These positions were called halfback, three-quarters back, and full back according to English and Scottish nomenclature and quarterback, halfback, and full back in the Irish nomenclature. In rugby the English-Scottish nomenclature was eventually adopted worldwide, with the word, "back", often omitted for brevity from the half-back ("half") and three-quarters back ("three quarter") names, and "fullback" as a single word.
In some systems, "five-eighths back" was added. (The illustration here, of singular forms, should not be construed as indicating the number of players in any of those positions, nor is the fraction in the name at all proportional to the actual depth of the position; they indicate only a form of ordinal number, not cardinality.) Having the backs at different depths facilitates passing movements in which the ball is tossed from one player to (usually) the next closest, such that each back receiving the ball, in turn, can be running forward and yet not ahead of the player who threw it, since forward passing of the ball is illegal. Because of the involvement of the 3/4 backs, such a movement is often called a three-quarters movement.
It was the Irish nomenclature of quarter back, half back, and full back that came to North America for use in what was to become the dominant native form of football. The terms became hyphenated and eventually unhyphenated single words, "quarterback" (QB), "halfback" (HB), and "fullback" (FB). The lack of quarterback in the English-Scottish nomenclature for rugby led to the position name "scrum-half" to distinguish the halfback playing close to scrimmage (renamed "scrummage" or "scrum") from another who would "stand off" from it or "fly" away—the "stand-off" or "fly-half".
Early Canadian football Around 1900, at the same time, the number of players on a Canadian football team was reduced to 14, and the number of players in scrimmage (formerly packing all the forwards) was fixed at three per team. It became customary for six to eight of the other players to continue as what might be known in rugby as loose (non-packing, i.e. not binding together) forwards who in Canadian football formed "winglines" on either side of the scrimmage. In scrimmage, each team had one centre scrimmager flanked by two side scrimmagers. The side scrimmager is bound with hands and arms to their centre scrimmager. The centre scrimmager of the side entitled (and required) to do so would put the ball down in front of him for play-by scrimmage, while both sets of three bodies each ("formed into one compact body" as the rules specified) were crouched and shoving forward at each other, probably meeting at the shoulders as do the front row of forwards in rugby's set scrummage. Depending on the rules, details of the time for the particular circuit of Canadian football clubs, the centre scrimmagers would either contend with their feet for the ball, or one would be entitled to foot it first (usually heeling it back), while the other teams would try to spoil the ball's delivery.
The backfield of three to five players continued to use the nomenclature (see above) of quarterback, halfback, and fullback, and sometimes included one or two flying wings. When six played on the winglines, their positions were called "inside wing", "middle wing", and "outside wing", arrayed in order away from the scrimmage. An additional player called "flying wing" could "fly", between a position on the wingline outside the outside wing, and the backfield.
However, the 14-a-side game's rules never required certain numbers of players in either the winglines or the backfield.
With the later reduction in sides from 14 to 12 players, and introduction of the thrown snap (not footed) snap, the positions of the three scrimmagers were replaced by a single position called "snap". The position names in the winglines were retained for a while but eventually replaced by the American nomenclature, although it was not until the 1960s, that Canadian rules required seven players on the offensive line, unless they were playing short handed. Before that, only five were required on plays in which no forward pass was thrown, so it is likely that in such situations, one or both ends were replaced by or became a back. "Snap" became "centre" and considered a wingline or line position. "Centre" even became the rule book designation of the snapper, although as in the American version, there is no necessity that the player who snaps the ball have equal numbers of teammates flanking him or her on the line.
More position names A tendency was seen, from early on, for players to keep their position names even when playing out of that position. For instance, before the minimum number of players on the offensive line was fixed at seven when players who would otherwise be in the line as guards or tackles played from offensive backfield positions, the formation was known as a "guard(s) back" or "tackle(s) back" formation.
Meanwhile, another position name was introduced. Players were placed in the offensive backfield just outside their ends, in a position that came to be called "wingback" (WB). Formations with one or two wingbacks came to be called single- or double-wing formations.
Over time, the typically fast back who played fullback, on offense and defense, was replaced by a heavier one who presented a greater threat to run with the ball more or less straight ahead, and to tackle his opponent trying to do the same. For this purpose, the player tended to be placed closer to the line than previously—often as far forward as the halfbacks (the offensive backs then forming a letter T, sometimes called a "straight T", later to distinguish it from slight variants wherein other backs did not form a line perpendicular to that from the quarterback) or even farther forward. But the position kept being called "fullback". On defense, this necessitated another position name for the fastback who played farthest back. The phrase "goal-tend" was used for a time near the start of the 20th century before settling upon "safety man" or simply "safety" (S), representing the last defense against a breakaway play, and the position from which to field opposing kicks. Approximately, on defense, the quarterback and fullback exchanged positions from what they'd been playing on offense, and the quarterback became the safety.
Still, when at the beginning of the 20th century, a penalty was introduced for hitting the opposing kicker after a kick, the foul was at first called "running into the fullback", inasmuch as the deepest back usually did the kicking.
Confusion increased when the legalization of the forward pass made it more advisable for teams to defend with fewer on the line of scrimmage and more behind. Although the offense could no longer play "guard back", for instance, the defense had no such limitations. A common defensive formation was the 6-2-3 or 6-2-2-1, shown below arrayed against an offense's straight T:
S HB HB C FB ____E_ T__ _G ____G_ T _ E____ E T G C G T E QB HB FB HB
The so-called center and fullback are playing left and right versions of the same position. The positions were later renamed "linebacker". However, that did not become the usual way to name that position until platoon football became common, with players specializing in offense or defense, substituted according to which team has the ball. There seems to have been some tendency for defensive positions to be named for where the player would play on offense, although quarterback and safety formed an early exception.
When linebackers are playing close to or level with the line, a somewhat reliable way to distinguish them from line players is that the defensive line players will be in a three- or four-point stance, meaning that they will be supported by one or both hands, in addition to feet on the ground, while the linebackers will have only feet on the ground. Line players with one or both hands on the ground are called "down linemen"; typically all defensive linemen are so positioned.
Alternate position naming schemes for an offensive formation are illustrated below in a version of a single-wing formation:
E G C G T T E QB HB HB FB
E T C G G T E QB FB WB TB
In the top illustration the center is flanked by guards in the 4-2 unbalanced line (four players on one side, the "strong side" of the center, two on the other, "weak", side) as in a balanced line, leaving the tackles together. In the bottom illustration, the guards are kept "inside" by being next to each other. The end positions are fixed by rule, because "end" is a rule book term.
As to the backfield, the top illustration preserves the principle of the names, reflecting distance from the line. However, the bottom illustration needs further explanation, having eliminated the halfbacks and introduced a new position name, "tailback" (TB). Wingback has been described above, but the "WB" in the above illustration is deeper back than that position name would seem to warrant. That is because this is a "wingback deep" version of the single wing, and the position is named wingback because the player is named for his true wingback position as it exists in other versions. In this version the player may stand as deep as any other back, but there has been a reluctance to identify formations in American football as having more than one fullback (or tailback—see below), although two-fullback systems were not uncommon at one time in rugby.
If the wingback were playing in a true wingback position but named halfback, then the other "halfback" would be playing deeper and thus deserve the name fullback. Another justification is that the player positioned as the left halfback in the top illustration tends to have the fullback characteristics described above. But rather than call the faster, lighter back behind (and in this formation, to the left of) him a halfback and so contradict the distance-back-determines-the-name principle (and rather than add rugby's three-quarters back), we call that player "tailback"—the tail of the formation, farther back than the fullback. Even when two players positioned to take a thrown snap in a single wing formation are level with each other (i.e. at equal depth), the faster one (usually playing weak side) is often called the tailback and the more massive one the fullback.