The chronograph is not a chronometer: it measures time intervals superimposed over the hours-minutes function without interfering with it. It's the distinction I always make first at the bench, because the confusion is nearly universal. The base movement continues to run; the chronographic mechanism engages and disengages above it when you press the button.
The heart of the complication is the command system. The column wheel — a star-profiled toothed drum — directs start, stop, and reset in sequence with a precision that flat-cam systems don't achieve. You recognize it by feel: pressure on the button has a sharp response, almost a clean click. In economical movements, cams are used because they cost less to mill; in serious calibers, the column wheel is used. The difference isn't aesthetic; it's in the repeatability of the gesture over time.
The other technical knot is the type of clutch. The horizontal clutch connects the seconds wheel to the central motion on the horizontal plane; at the moment of engagement, it can cause a micro-jump of the hand. The vertical clutch couples the components on a vertical axis and gives a start practically free of jolts. In modern integrated calibers, the vertical clutch has become almost a standard; in many vintage chronographs and in certain modular calibers, you find it horizontal, and the small jump of the hand at start is a useful diagnostic sign during an overhaul.
On button layout: the classic two-button chronograph has start/stop at 2 o'clock and reset at 4 o'clock. The single-button — often integrated into the crown or a single pusher — cycles the three functions in sequence with successive presses; it requires attention in use because you cannot stop and restart without passing through zero. Flyback solves the opposite problem: with the chronograph running, a single press resets and restarts instantly — useful for consecutive measurements without interruption. Longines developed it in the Thirties for aviation; today you find it in integrated calibers of proprietary production and in some Dubois-Dépraz modules.
The great chronographic calibers from history that I've disassembled most often: the Valjoux 72, the Venus 178, the Landeron 248, the Caliber 11 modular (based on Calibre 11 with Dubois-Dépraz, crown at 9 o'clock as on the Monaco), and Universal Genève calibers — the 281 of the Compax and the 481 with full calendar. The Excelsior Park EP40 manual-wind is undervalued: the button mechanism is among the most precise in the category, better than the Valjoux 72 in most of the examples I've handled. The chronograph belongs, along with the minute repeater and the perpetual calendar, among the three complications traditionally recognized as grand complication.