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.

Being variants of 19th century rugby football, 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".

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.