Toe the line
To conform to an established standard or political programme.
There is some confusion between 'toe the line' and the frequently seen misspelling 'tow the line'. The 'tow' version is no doubt encouraged by the fact that ropes or cables on ships are often called lines and that 'tow lines' are commonplace nautical items.
The earlier meaning of 'to toe the line' was to position one's toes next to a marked line in order to be ready to start a race, or some other undertaking. In the 19th century, we wouldn't have been limited to lines when it came to placing our feet, but would have had a choice of what to toe - a mark, scratch, crack or trig [a line or small trench]. These were all then in use in 'toe the ...' phrases. The earliest version we know about is from The Diverting History of John Bull and Brother Jonathan, 1813, by 'Hector Bull-Us' - known to his family and friends as James Paulding:
"He began to think it was high time to toe the mark."
Pauling was using the figurative rather than literal meaning of the phrase, i.e. to 'toe the mark' was to conform to a set standard.
Going back to the original, literal 'toeing' of a line; there are many circumstances where one might place one's toes up to a line - the start of a sporting event, standing in formation on parade, etc, etc. So, which is the source of the phrase?
One explanation that is often repeated is that the phrase derives from the British House of Commons. Arguments in the House are often heated. To deter members of opposing parties from attacking each other, two parallel red lines are marked, two sword-lengths apart, on the floor of the house. MPs are expected to stay behind these lines when a speech is in progress. Members, of course, no longer carry swords, but the tradition remains. Visitors to the House of Commons are very likely to hear this tale related by a tour guide. Counting against this supposed derivation is the fact that the current Commons Chamber dates from only 1950, when the building was rebuilt following WWII bomb damage. Paintings of earlier Commons chambers, from the times when members might actually have worn swords, show no such lines. The parliamentary link may be strengthened in some people's minds because of the 'toe the party line' usage, which relates to orthodoxy in politics.
Another possible source is prizefighting. The scratch was the line marked across the ring in early 'toe-to-toe' boxing bouts. Anyone man enough to enter into such a contest was 'up to scratch' (see also: start from scratch). This version of the phrase was known in the USA by the early 19th century, for example, this piece from the Gettysburg newspaper The Peoples Press, from October 1835, in which the public was invited to put up or shut up in a wager about an election:
Come gentlemen "toe the scratch" or hereafter forever hold your peace.
Other early examples of 'toe the ...' have a nautical connection. In the 19th century, sailors were expected to prepare themselves for group punishment by standing in formation on deck and 'toeing the line' between boards - also called 'toeing the crack'. This usage is the earliest that I've found for 'toe the line' in print - from The Edinburgh Literary Journal, January - June 1831:
"The matter, therefore, necessarily became rather serious; and the whole gang of us being sent for on the quarter-deck, we were ranged in a line, each with his toes at the edge of a plank, according to the orthodox fashion of these gregarious scoldings, technically called toe-the-line matches."
Which is the source? Well, no one knows. What is for certain - it is toe, not tow.