Loosehead prop and tighthead prop

In the game of rugby union, there are 15 players on each team, comprising eight forwards (numbered 1–8) and seven backs (numbered 9–15). In addition, there may be up to eight replacement players "on the bench". Jersey numbers 16–23 differentiate them. Players are not restricted to any single position on the field, although they generally specialise in just one or two that suit their skills and body types. Players that specialise in over three positions are called "utility players".

The scrum (an assemblage used to restart play), however, must consist of eight players: the "front row" (two props, a loosehead and tighthead, and a hooker), the "second row" (two locks), and a "back row" (two flankers, and a number 8). The players outside the scrum are called "the backs": scrum-half, fly-half, two centres (inside and outside), two wings, and a fullback.

Early names, such as "three-quarters" (for the wings and centres) and "outside-halves" (for fly-half) are still used by many in the Northern Hemisphere, while in the Southern Hemisphere the fly-half and inside centre are colloquially called "first five-eighth" and "second five-eighth" respectively, while the scrum-half is known as the "half-back".

The backs play behind the forwards and are usually more lightly built and faster. Successful backs are skilful at passing and kicking. Full-backs need to be good defenders and kickers, and have the ability to catch a kicked ball. The wingers are usually among the fastest players in a team and score many of the tries. The centres' key attacking roles are to try and break through the defensive line and link successfully with wingers. The fly-half can be a good kicker and generally directs the backline. The scrum-half retrieves the ball from the forwards and needs a quick and accurate pass to get the ball to the backs (often firstly to the fly-half). Forwards compete for the ball in scrums and line-outs and are generally bigger and stronger than the backs. Props push in the scrums, while the hooker tries to "hook" the ball. Locks are tall and jump for the ball at the line-out after the hooker has thrown it in. The flankers and number eight should be the first forwards to a tackle and play an important role in securing possession of the ball for their team.

There are a maximum of 15 players from each team on a rugby field at one time. The players position at the start of the game is indicated by the number on the back of their shirts, 1 to 15. The positions are divided into two main categories; forwards (numbered 1 to 8) and backs (numbered 9 to 15). In international matches, there are eight substitutes that can replace an on-field team member. The substitutes, numbered 16 to 23, can either take up the position of the player they replace or the on-field players can be shuffled to make room for this player in another position. Typically, the replacement players will have a number that corresponds with their intended replacement position with the numbers from 16 to 20 being forwards and 21 to 23 being backs (depending on the composition of the reserves). There are no personal squad numbers and a versatile player's position and number may change from one game to the next. Players can also change positions with players on the field during the match, and, as long as the laws are followed, any player can change positions with another player during the match. Common examples are the fly-half playing the full-back's position in defence or a prop taking the hooker's position at line-outs.

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Wikipedia