A rugby union team is made up of 15 players: eight forwards, numbered from 1 to 8; and seven backs, numbered from 9 to 15. Depending upon the competition, there may be up to seven replacements.
Each player has a fixed role with specialist positional skills and each team uses the same formation, with only minor variations; in this respect it is different from both football with its various formations (4-3-3, 3-5-2, etc.) and cricket, where players are commonly moved from one field position to another (e.g. from silly mid-on to deep cover point).
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 counter-attacking 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 tends as "half-tends". It being 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 became more sophisticated, backs positioned at different depths (i.e. distances behind the forwards) were further differentiated into half back, three quarters (the fraction 3/4) back, and full back, according to British nomenclature, which 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.