that, which

   To understand the distinctions between that and which it is necessary to understand restrictive and nonre-strictive clauses. Learning these distinctions is not, it must be said, anyone's idea of a good time, but it is one technical aspect of grammar that every professional user of English should understand, because it is at the root of an assortment of grammatical errors.

   A nonrestrictive, or nondefining, clause is one that can be regarded as parenthetical: "The tree, which had no leaves, was a birch." The italicized words are effectively an aside and could be deleted. The real point of the sentence is that the tree was a birch; its leaflessness is incidental. A restrictive, defining, clause is one that is essential to the sense of the sentence. "The tree that had no leaves was a birch." Here the leaflessness is a defining characteristic; it helps us to distinguish that tree from other trees.

   In correct usage that is always used to indicate restrictive clauses and which to indicate nonrestrictive ones. Restrictive clauses should never be set off with commas and nonrestrictive clauses always should. On that much the authorities are agreed. Where divergence creeps in is on the question of how strictly the distinctions should be observed.

   Until relatively recently they were not observed at all. In the King James Bible, for instance, we find "Render therefore unto Caesar the things which are Caesars; and unto God the things that are Gods." The same quotation appears twice more in the Bible-once with that in both places and once with which in both. Today, that is more usual in short sentences or early on in longer ones ("The house that Jack built"; "The mouse that roared"). Which often appears where that would more strictly be correct, particularly in Britain, as here: "It has outlined two broad strategies which it thinks could be put to the institutions" (Times).

   Although there is ample precedent for using which in restrictive clauses, the practice is on the whole better avoided. At any rate, on some occasions the choice of which is clearly wrong, as here: "On a modest estimate, public authorities own 100,000 houses, which remain unoccupied for at least a year" (Sunday Times). What the writer meant was that of those houses that are publicly owned, at least 100,000 are left vacant for a year or more. Deleting the comma after houses and changing which to that would have made this immediately clear.

   Another common fault-more a discourtesy to the reader than an error-is the failure to set off nonrestrictive clauses with commas, as here: "Four members of one of the worlds largest drug rings [,] which smuggled heroin worth £5 million into Britain [,] were jailed yesterday" (Times). That lapse is seen only rarely in America but is rife in Britain; it occurred five times more in the same article.

   Americans, in contrast, are much more inclined to use that where which might be preferable, as here: "Perhaps, with the help of discerning decision-makers, the verb can regain its narrow definition that gave it a reason for being" (Safire, On Language). Had Safire written "can regain the narrow definition that gave it a reason for being," all would be well. But the use of "its" gives the final clause the feel of a nondefining afterthought, and the sentence might be better rendered as "can regain its narrow definition, which gave it a reason for being." The point is arguable.