Watch movement terms explained

How a movement works

A mainspring stores energy. A gear train carries it to the escapement, which lets it out in tiny, even steps. The balance wheel and its hairspring swing back and forth at a steady rate, and each swing lets the hands move on a little. The rest of the vocabulary describes one of those parts.

On the spec sheet

• Jewels: synthetic rubies used as bearings where parts turn. A simple automatic has about 21 to 25. More jewels is not better beyond that; they usually mean more complications.
• Beats per hour (bph) or frequency: how fast the balance swings. 21,600 bph (3 Hz) is common on affordable watches, 28,800 bph (4 Hz) on most modern Swiss ones, and 36,000 bph (5 Hz) on high-beat watches. Higher can be steadier, and the seconds hand sweeps more smoothly.
• Power reserve: how long it runs fully wound. Around 40 hours is common; many newer movements reach 70 to 80. More on power reserve.
• Hacking: the seconds hand stops when you pull the crown, so you can set it to the second. More on hacking.
• Hand-winding: whether an automatic can also be wound by the crown. Most can; some cheaper movements cannot.

On a timegrapher

• Rate: seconds gained or lost a day. What is normal.
• Amplitude: how far the balance swings, in degrees. Around 270 to 310 degrees fully wound is healthy; low amplitude often means it needs a service.
• Beat error: how evenly the balance swings each way, in milliseconds. Under about 1 ms is good.

On the marketing

• COSC: the Swiss chronometer test, -4 to +6 seconds a day for the uncased movement.
• In-house: designed and made by the brand itself, rather than bought from a supplier.
• ETA, Sellita, Miyota, Seiko: the big movement makers whose movements are in many brands' watches. Proven and easy to service, which is not a downside.
• Silicon hairspring: resists magnetism and temperature changes. Look for it if you work near magnets.