The origin of this idiom is disputed, and some think it came from the British House of Commons where members were required to stand two sword-lengths apart to maintain decorum (and apparently to keep members from being able to reach someone arguing with them with their sword). However, Wikipedia believes it is more likely that the origin was barefooted seamen standing at attention for inspection lined up along the seams of wooden planks on the deck, “…hence, to ‘toe the line’.” Others attribute it to the starting line in track events and boxers lining up on either side of a line to start a match.