What it solves
In a normal mechanical watch, gravity affects the balance wheel's rate differently depending on which way the watch is held. Dial-up, dial-down, crown-up, crown-down, crown-left, crown-right; each of these positions changes how friction loads the balance pivot, and the watch runs slightly faster or slower in each. Pre-1801 pocket watches were carried in vest pockets in one fixed vertical position, so this rate variation could add up to noticeable error over 24 hours.
The tourbillon mounts the entire escapement assembly inside a small rotating cage. The cage rotates through a full 360° once per minute (or per cage-rotation period; some go 4 minutes, 6 minutes, 24 minutes). Across each rotation, the balance wheel spends equal time in every vertical position, so positional rate errors average out to roughly zero. In a stationary pocket watch this is genuinely useful.
Why it doesn't matter much in wristwatches
Modern wristwatches sit in many positions over a normal day (you move your wrist, sleep with the watch dial-up or dial-down, leave it on the dresser overnight). The "averaging" effect that a tourbillon does mechanically already happens naturally just from the watch's daily movement. Modern hairspring technology (silicon, Nivachron, Spiromax) handles positional errors better than 19th-century materials could. So a chronometer-grade non-tourbillon (Rolex Cal. 3235 at -2/+2 sec/day) outperforms most tourbillons in real-world wear.
Why brands still make them
Three reasons. First, visual drama: the spinning escapement is mesmerising and is now a recognisable status signal. Second, finishing showcase: a tourbillon cage requires hand-finishing the bridge, the cage itself, and the movement around it; it's a flex of haute-horlogerie capability. Third, brand prestige: a tourbillon is the cheapest way to add a "grand complication" to a maker's catalogue. Below CHF 50,000, the watch is mostly buying you the badge; above CHF 200,000, you start getting genuinely difficult finishing.
Modern variations
Flying tourbillon: the cage is supported only from below (no upper bridge), making the cage appear to float. Patented by Alfred Helwig in 1920 for German watchmaking schools. Multi-axis tourbillon: rotates around two or three perpendicular axes simultaneously (Greubel Forsey 24-Secondes, Jaeger-LeCoultre Gyrotourbillon). Genuinely averages every conceivable position. Fast-rotating tourbillon: rotates faster than 60 seconds (down to 6 seconds in some Greubel Forsey pieces) for theoretically tighter averaging. Beautiful, expensive, mostly a flex.
What you get for the money
CHF 30,000 buys you an Asian-tier industrial tourbillon (Memorigin, Beijing Watch Factory, often unbranded under low-cost-luxury labels). CHF 50,000-150,000 buys a Swiss industrial tourbillon (Hublot, Zenith, Bulgari, Blancpain). CHF 200,000+ buys haute-horlogerie tourbillon (Patek, AP, Lange, F.P. Journe, Greubel Forsey). At every tier you are paying for finishing and brand; the tourbillon's functional contribution is marginal in modern wear. See /styles/tourbillon/ for current tourbillon news.