In the immortal words of Harry Callahan, I know what you’re thinkin’ — but before you sniff, say to yourself, “For God’s sake, man, I know what a tourbillon is, don’t insult my intelligence,” and turn the page, lend me your ear, I prithee — I’m going somewhere with this.

Most discussions of the tourbillon focus not on what it is physically, but on what it does; and of course the odds are pretty good that if you’ve picked up this magazine at all, you’ll know that Breguet invented the tourbillon, patented it in 1801, that it’s intended to create a single averaged rate error in the vertical positions and so simplify the task of matching the vertical rate to the flat rates of the oscillator, thus making the watch immune to positional rate variations. (Those who object to the tourbillon do so on the grounds that it eliminates no error at all — that it merely smears together the four different errors in the four different vertical positions into a single mélange of imprecision, and that four wrongs don’t make a right — but that’s another argument.)

But what makes a tourbillon watch a tourbillon watch? The question came up during an exchange on that most unsuitable medium for rational discourse, Twitter, when several of us got into a brouhaha over what constitutes a double tourbillon — and, specifically, whether the Greubel Forsey Double Tourbillon 30° is a “real” double tourbillon.

For comparison, one of the participants in this 140-character horological symposium instanced the Roger Dubuis and Breguet double tourbillons — two separate cages, two balances, two escapements. Surely these, went the thesis, were examples of “real” double tourbillons.

To know what a double tourbillon is, it seemed to me, one ought to think about what a tourbillon is — that is, what makes a tourbillon watch a tourbillon watch and not something else.

Despite the lamentably freewheeling tendency to use the term “tourbillon escapement” (which can be found in professional reference texts going back generations; my 1946 copy of Britten’s The Watch & Clock Maker’s Handbook uses the terms “tourbillon” and “tourbillon escapement” virtually interchangeably) it seems to me that the tourbillon, strictly speaking, is not an escapement, but a regulating mechanism. To illustrate this, one need merely consider that a tourbillon watch may be fitted with any of a number of different escapements: there are tourbillon watches with lever escapements, side-lever escapements, chronometer-detent escapements, co-axial escapements, and so on.

A watchmaker friend challenged me on the subject of a double tourbillon necessarily meaning two oscillating organs — two balances. “Is a tourbillon a tourbillon without a balance?” he asked, rhetorically. My reply was that we can refer to the components of a watch — composite or single — without requiring that the balance be thought of as a part of that component. A mainspring barrel, out of a watch, disconnected from the balance, is still a mainspring barrel.

It then occurred to me to ask someone who should know, if anyone does: Stephen Forsey, of Greubel Forsey, whom I’d last seen in Geneva during SIHH (he was gamely nursing a badly injured back and was both introducing the Garde Temps project as well as the new Greubel Forsey GMT watch). An email to Stephen at the manufacture sat in limbo for a few days, but in the fullness of time, Stephen responded that a tourbillon is composed of a fixed wheel and a functionally distinct, rotating cage or carriage.

The definition is reductive and accurate — after all, it is only the presence of those parts that makes a watch a tourbillon. Without the cage and fixed fourth wheel, one simply has an ordinary watch.

Greubel Forsey’s “double tourbillon” is therefore certainly a real double tourbillon — it is simply that it is one with a single oscillator, unlike double tourbillons made by Roger Dubuis and Breguet, for instance, in which there are two regulating systems (two tourbillons, two escapements and two oscillators) linked by a differential. The habit of referring to the tourbillon as a “tourbillon escapement” has been decried by some experts as sloppy; I once regarded this attitude as egotistically self-serving hairsplitting from individuals over-invested in being perceived as experts — but having thought about it, I think I see the point. The tourbillon is no more an escapement than is a remontoir or chain-and-fusée — both, like the tourbillon, intended to improve rate stability (over power reserve rather than across positions, but the basic purpose is the same).

Fine distinctions? Sure. But without such niggling semantic debates, as one of my favorite watch writers, Kenneth Ulyett, put it, “keen horologists would be deprived of the pleasure of arguing with each other”.