Of all the marks of English language punctuation, the semicolon--;--is perhaps the least used and most misunderstood. It's favored, to my knowledge, by only three groups of wordsmiths: newspaper headline writers, who use it to change subjects in mid-headline; lawyers, to introduce caveats, and legislators (often lawyers themselves) who use it to spell out legal exceptions (" . . . this shall be done; however, under some conditions it shall not . . . .").

A dispatch from Richmond now informs us that a law passed by the recent General Assembly has been found to contain a flaw; it includes a semicolon that changes much of its meaning.

The semicolon appeared in a bill dealing with state roads; it was introduced by Del. Alan Diamonstein (D-Norfolk). Its purpose was simple; it added 22 words to a law controlling how roads qualify for state-financed maintenance.

The words in the law were accurate; however, a semicolon somehow inserted later in the measure managed inadvertently to nullify requirements for road drainage and cancels the obligation of counties to share the cost of bringing such roads up to state standards.

It is uncertain how many substandard roads might qualify for addition to the state road system under the new law, officials said; one of them, J.T. Warren, director of administration for the state Highway Department, observed that "the thing probably needs to be redrafted." To reproduce the legislation would take the rest of this column; in fact, probably even more.

Gov. Charles S. Robb's staff has not recommended yet whether the governor should sign the bill; there has been, however, no threat of a veto.