The Washington Times recently printed an editorial about H1N1 flu calling the disease “tow-the-line flu”. The use of the phrase “tow the line” is a common mistake; what the paper should have written was “toe the line”. To “toe the line” means to conform to some rule or standard, to fall into line. Politicians, for example, often have to toe their party lines.
People may imagine that the spelling “tow the line” is correct as it perhaps derives from some nautical activity. Ropes are often called lines aboard ship and a tow-line is just a line used to tow something on the water. But the phrase is probably nothing to do with ropes. In fact, the exact source is unclear but the phrase is generally taken to derive from the idea of lining up for a sporting activity, i.e. to place your toe on the line for the start of a race. By doing so you are following the rules set out for the activity.
There are other theories as to the origins of the phrase. It may derive from boxing, with early prize-fighters having to stand with one foot on a scratched line on the ground to fight. Others have claimed that it derives from the British House of Commons, where lines are marked on the ground to prevent more adversarial debates from getting out of hand.
Whatever the true origin of the phrase, the spelling should be “toe” and not “tow”.