How it works
A chronograph adds a second time-keeping mechanism to the watch, layered on top of the regular hours/minutes display. The chronograph mechanism has its own clutch (engaging when you press the start pusher), its own seconds hand on the centre, and one or two sub-dials counting elapsed minutes and hours. Press start, the centre seconds sweep begins. Press start again, it stops. Press the reset pusher, all hands snap back to zero.
The classical layout: pusher at 2 o'clock = start/stop. Pusher at 4 o'clock = reset. Sub-dial at 9 = small running seconds for the regular time. Sub-dial at 3 = elapsed minutes (typically 30-min counter). Sub-dial at 6 = elapsed hours (typically 12-hour counter). The arrangement varies; some chronographs put running seconds at 6 and use 9 and 3 for minutes/hours.
The two engineering generations
Manual chronographs dominated until 1969. The Valjoux 72, Lemania 2310 / 2320, Venus 175, and Universal Genève 285 are the canonical mid-century manual chronograph movements. The 1957 Omega Speedmaster uses Lemania 2310 / Cal. 321 (later 1861, now 3861); the 1963 Rolex Daytona originally used Valjoux 72 before moving to in-house Cal. 4030 (1988) and Cal. 4130 (2000).
The 1969 automatic chronograph race opened the modern era. Three competing groups (Project 99 / Heuer-Breitling-Hamilton-Buren-Dubois-Dépraz with Cal. 11; Zenith with the El Primero; Seiko with Cal. 6139) launched the first automatic chronograph movements within months of each other. The Valjoux 7750 (1973, Edmond Capt) became the volume Swiss workhorse and remains so today; its descendants power most chronographs under CHF 10,000.
Modern tiers
Volume Swiss tier (CHF 1,500-8,000): Valjoux 7750 / ETA 7750, Sellita SW500 chronographs. The mechanism every brand without an in-house chronograph uses. Tag Heuer Cal. 16, Tudor 41 chronograph, Hamilton Intra-Matic, Longines Master Collection.
In-house manufacture tier (CHF 12,000-50,000): Rolex Cal. 4130 (Daytona), Omega Cal. 9900, AP Cal. 4400, Heuer 02, Patek CH 28-520. Built end-to-end by the brand; better finishing, often integrated rather than modular construction.
Haute-horlogerie tier (CHF 50,000+): Patek CH 29-535 PS column-wheel chronograph, Lange Datograph (L951.1), AP Cal. 2385 chronograph. Hand-finished, observable through display caseback, the genre's ceiling.
Specialist variants
Flyback chronograph: a single press of the reset pusher resets AND restarts the chronograph in one motion (instead of stop-reset-start, three presses). Originally a pilot-watch convention for in-flight timing changes. Split-seconds (rattrapante): two centre seconds hands; one stops while the other continues, useful for timing two events from a common start. Monopusher: a single pusher does start/stop/reset on successive presses. The vintage and haute-horlogerie aesthetic.
See: /styles/chronograph/ for current chronograph news, wiki: chronograph for the full reference.