While the phrase has been indisputably re-used several times throughout history, from naval ships, to boxing, to foot races, the most convincing origin of the phrase comes from the House of Commons in British Parliament.
The House of Commons, historically and currently in its modern form, has two sets of lines separating the front row benches. These lines are better than a "sword's length" apart from each other, to keep over excited members from appealing to their swords to settle debate. It was quite common through much of the House's history, dating back to the 14th century, to have its members armed with swords.
In modern courtrooms you may hear a judge demand "order, order in the courtroom" in times of heated exchange. In the House of Commons, the Speaker would demand that members, "Toe the line, toe the line," if debate was becoming heated, particularly along the front rows. The mortal consequences of heated exchange between armed men demanded strict adherence to the House rules. Thus, the phrase “toe the line” was echoed throughout the House to return order and to quell the growing conflict.
The primary connotation of “toe the line” is: “To adhere to rules or doctrines conscientiously; conform” (American Heritage) and “To conform to a rule or standard” (Oxford). Thus “toeing the line” was conforming to the rules of the House of Commons, just as maintaining “order” is conforming to the rules of a courtroom.
A visit to the House of Commons at The Palace of Westminster will confirm this version of the idiom’s history and they will proudly show you the two lines running through the hall.
The most commonly cited source for the “sports origin” theory is foot-racing, where the competitors must keep their feet behind a "line" or on a "mark" at the start of the race—as in "On your mark, get set, go!" So one who "toes the line" is one who does not allow his foot to stray over the line. Another sports theory is boxing, where two boxers were required to stand toe to toe with one another on a line. A referee would call out, "Toe the line!" requiring both boxers to put their respective toes on a chalk line, face each other and get ready to box.
Sometimes this phrase is written "tow the line." This misspelling changes the meaning of the phrase slightly: rather than implying conformance with a rule, "tow" suggests contribution to a cause, e.g. "the pundit is towing the administration's line" alluding to a metaphorical act of pulling something with a line, cord or rope. However, this variant is grammatically suspect, as the verb tow refers to the object being towed, e.g. a car or a boat, not the mechanism by which it is towed, such as a rope or chain.
The phrase has also come into usage in the United States military, as a command for recruits to come to attention in a designated area (to literally place the toe on an imaginary line).
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