Here is the whole argument settled in a single line: all cognac is brandy, but not all brandy is cognac. Brandy is any spirit distilled from fermented fruit, usually grapes. Cognac is brandy made in one small corner of western France, under rules strict enough to make a Swiss banker feel relaxed about his own paperwork.

It is one of those questions that sounds like a trap and turns out to be genuinely useful, the drinks-cabinet equivalent of squares and rectangles. Get it straight once and you will never again stand in front of a shelf, or a restaurant's after-dinner list, feeling quietly outranked. So let us get it straight.

So what exactly is brandy?

Brandy is the big, generous family name. Take any fruit, ferment it into something wine-ish, then distil that wine to concentrate the alcohol and the flavour, and you have brandy. Grapes do most of the heavy lifting around the world, but apples make the glorious brandies of Normandy, pears and plums have their moments, and every wine-growing country worth its salt has a local version. Spain has its sweet, raisined brandies from Jerez; Peru and Chile have pisco. Normandy has Calvados, the apple brandy that tastes like an orchard in late October, and a bottle like the Busnel Calvados Pays d'Auge is brandy to its bones, without a drop of grape in sight. Gascony, meanwhile, quietly turns out Armagnac, France's older and wilder grape brandy, the sort of thing collectors hoard by the vintage.

And what makes cognac, cognac?

Cognac is brandy that has passed an exam. To earn the name it must be made in the Cognac region, the gentle, chalky sweep of countryside north of Bordeaux around the river Charente. It must be distilled mostly from Ugni Blanc, a thin, sharp white grape that makes thoroughly mediocre table wine and, as luck would have it, magnificent spirit. It must be double-distilled in copper pot stills, the beautiful onion-shaped alembics that have barely changed in centuries. And it must age in French oak, where the raw spirit slowly trades its fire for colour, roundness and that trademark note of dried fruit and warm forest floor. Houses like Hennessy and Rémy Martin have been doing precisely this, with almost monastic consistency, for the better part of three hundred years.

What do VS, VSOP and XO actually mean?

This is the code everyone pretends to understand and quietly does not. It is simply an age ladder, set by the youngest spirit in the blend. VS, for Very Special, means the eau-de-vie has rested at least two years in oak; it is bright, fruity and built for mixing, which is exactly what a Hennessy VS at £39.99 is for. VSOP, Very Superior Old Pale, lifts the floor to four years and lands in the sweet spot where a cognac is smooth enough to sip but still has some snap, and a Rémy Martin VSOP at £52.99 is the classic of the type. XO, Extra Old, now means at least ten years, and it is where cognac turns plush and contemplative, all candied orange, leather and baking spice. A Hennessy XO at £195.99 is a bottle for the end of a very good evening, not the middle of a Tuesday.

Is cognac always the better bottle?

No, and anyone who says otherwise is selling something. Cognac is a style, not a scoreboard. A great Armagnac can out-swagger a middling cognac any day of the week, with more rusticity and a vintage-dated soul, and the Domaine d'Aurensan 1998 Armagnac is proof that single-year grape brandy can move you as much as the big Cognac names, just with mud on its boots. A properly made Calvados brings something cognac never can, that cold, bright snap of orchard fruit. The honest way to hold it in your head: cognac is the polished, dependable classic, and the wider brandy world is where the eccentrics and the real bargains live. You want both within reach.

So which should you actually buy?

Start with where you are pointing the bottle. If it is going into a Sidecar or a Brandy Alexander, or simply over ice with a twist, do not overthink it. A VS cognac like Hennessy, or the excellent-value Cognac Philbert VS at £38.99, has all the fruit and backbone a cocktail needs. If you want something to sip by the fire, step up to a VSOP and give it room in a glass with a bit of belly. And if you are buying a gift, or marking an occasion, an XO or a vintage Armagnac says what a card never could. The only real mistake is drowning a ten-year-old XO in a mixer. That is not confidence, that is vandalism.

However you drink it, treat the glass kindly. Brandy wants a little warmth to open up, so skip the deep freeze and cup it in your hand for a minute or two. Neat is the purist's move, a single ice cube is nobody's crime, and a good cognac in a Sidecar is one of the great underrated cocktails of the twentieth century, quietly waiting for its comeback.

So the next time an after-dinner list arrives and someone asks whether you fancy a brandy or a cognac, you can smile, because it was always a gentle trick question. Order the cognac if you want the classic. Order the Armagnac if you want the story. Either way you are drinking brandy, and either way you are drinking well.