19.13 OFFSIDE LINES AT THE LINEOUT

(a) When a lineout forms, there are two separate offside lines, parallel to the goal lines, for the teams.
(b) Participating players. One offside line applies to the players taking part in the lineout (usually some or all of the forwards, plus the scrum half and the player throwing in). Until the ball is thrown in, and has touched a player or the ground, this offside line is the line of touch. After that, the offside line is a line through the ball.
(c) Players not taking part. The other offside line applies to the players not taking part in the lineout (usually the backs). For them, the offside line is 10 metres behind the line of touch or their goal line, if that is nearer.