Toe the line

Several institutions each had a literal line for people to put their toes on, and any of them could be the true origin. Runners lined up with toes at a starting line before a race. Soldiers on parade were drilled to stand with toes exactly aligned along a mark on the ground. The British House of Commons is popularly (though shakily) said to have chamber floor lines set two sword-lengths apart that members were not to cross during debate. The military drill and athletics senses both have solid 19th-century citations; the swords-in-Parliament version is much harder to verify.