Patek Philippe CH 29-535 PS Mode D'emploi page 7

CHRONOGRAPH
The chronograph is started and stopped with the
pusher at 2 o'clock. The chronograph and minute-
counter hands are reset to zero with the pusher
at 4 o'clock.
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TACHYMETER
Your watch has a tachymeter scale calibrated to
1000 meters . On a highway, a car passenger
would typically start the chronograph when passing
a kilometer sign and stop it when passing the next
one. The chronograph hand shows the elapsed time
between the signs and, on the tachymeter scale, the
average speed of the car during the measurement. It
therefore measures the time needed to travel a given
distance rather than the distance travelled within a
given time.
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