I have a question that I’m hoping that you can answer on both a practical basis, and a historical basis.
If the lines are part of the area that they surround, and the ball is out of play when the whole ball crosses the whole line, then why is it that a player can stand on the line while taking a throw-in? It doesn’t seem to make any sense. I would think that if the ball is out of play over the touch line, that the thrower would have to stand completely off the field, and then throw the ball back onto the field.
I’m sure that there is a practical and historical reason for this, I just don’t know what it could be.
USSF answer (March 30, 2009):
We, too, are intrigued to know the answer, but we were unable to find anything in writing. However, a noted historian of the Laws of the Game suggests that a practical reason for requiring the thrower to stand on or outside the touch line is to help localize the point where the ball left the field. It is intended to discourage a throw from several yards away from the line and the ball entering the field far from the correct entry point.
As a further contribution to the historical side, the two-handed throw-in from the touchline developed after a compromise settlement between varying sets of rules, some of which allowed single-handed throws. This occurred at the first IFAB meeting in December 1882, and resulted in a two-handed throw in any direction. In 1895 throwing distance was restricted by a rule compelling the thrower to stand with part of both feet on the touchline. The rule was changed so that the thrower’s foot had to be outside the touchline (1925) or on or outside the touchline (1932), which is the rule today.
For further information on the throw-in and other items related to the Laws and customs of the game, see “Ward’s Soccerpedia,” a history of The Lore and Laws of the Beautiful Game, by Andrew Ward.