The movements are not pictures of movements. Each caliber is laid out from its own tooth counts and pitch circles, and everything you can do to it follows from that geometry.
I
The train decides the beat
Barrel to centre to third to fourth to escape, each pair meshing at the sum of its pitch radii. Count the escape wheel's teeth, multiply by its turns an hour, double it for the two beats a tooth: that is where 18,000 an hour comes from, and the tick you hear is that number.
II
Parts cover each other by their real shapes
Nothing comes off in a scripted order. A part lifts when its own footprint and height leave it clear, so the balance comes off before the pallet cock and the ratchet before the barrel bridge — and when a part will not move, the game tells you which piece is holding it.
III
The fault is found by its symptom
A broken mainspring lets the crown turn and turn with nothing catching. A rusted click spring lets it spin back. A loose cannon pinion leaves the hands standing while the train runs. Old oil at the balance stops the watch face down and not face up.
IV
Oil goes on before the part that covers it
Each jewel takes the oil it takes — thin for the fast pivots, heavy for the slow ones, pallet stone oil on the exit stone, grease on the keyless works — and a drop you forget is a drop you will be taking a bridge off to reach.
V
The timegrapher is the escapement
Every dot on the trace is a beat the simulation actually made. A slope is a rate, a gap between the lines is a beat error, and a watch out of beat limps audibly before you ever put it on the machine.
VI
The shelf keeps running
A finished watch goes on its stand and keeps its own time from its own mainspring, running down after about a day and a half as it would. Pick one up, wind it, set it, open its back and look at the movement you rebuilt.
