Splitting the Difference

I think they meant one of these. Or two of these. Hard to tell.

There's a twisted, irresistibly delightful site that you definitely ought to visit called Pimp My Snack, in which a batch of Brits make enormous versions of ordinarily tiny snack food such as Cadbury's creme eggs and Kellogg's Nutri-Grain cereal bars.

On the second page of their three-page attempt to gigantify a Peak Frean's Bourbon cookie appears the following paragraph:

Step 3 &4 - the lettering was then printed with the centre marked on it. Then it was placed on the slab and a pair of compasses used to accurately mark the positions of the letters:

"Pair of compasses"? Why, you may be asking yourself, as I briefly did, would they need two of them, when one compass ought to do the trick nicely (and a divider would probably work even better)? And then I remembered that Britons say "pair of compasses" where we North Americans just say "compass". We've lost the plural-ness of that compass, even though we've retained it for such things as tweezers, [eye]glasses, and pliers. (We also don't say "a pair of dividers", but, once again, the Britons do.)

Conversely, Americans sometimes call a pair of scissors "a scissors", which I've never ever heard in Canada and don't ever expect to. As a further example of random perversity, the fashion industry has a jarring insistence on dropping the "pair of" from certain articles of clothing; it's strange to hear Stacy London describe something as "a pant".

All we can glean from this information is that 1) objects which are bifurcated--which have two "legs"--are, or were, referred to as a pair of something, 2) sometimes we just drop the twoness, and 3) there's no rhyme nor reason to any of it, which is just to say it's a microcosm of the language as a whole.