SCREW BALANCE
Before the invention of the perfectly weighted balance using a smooth ring, balances were fitted with weighted screws to get the exact impetus desired. Today a screw balance is a subtle sign of quality in a movement due to its costly construction and assembly utilizing minuscule weighted screws.

SKELETONIZATION

The technique of cutting a movement's components down to their weight-bearing basic substance. This is generally done by hand in painstaking hours of microscopic work with a mini hand held saw, though machines can skeletonize parts to a certain degree.

SONNERIE
A variety of minute repeater that---like a tower clock---sounds the time not at the will of the wearer, but rather automatically (en passant) every hour (petite sonnerie) or quarter hour (grande sonnerie).

SPLIT-SECONDS CHRONOGRAPH
Also known in the watch industry by its French name, the rattapante. A watch with with two second hands, one of which can be blocked with a special dial train lever to indicate an intermediate time while the other continues to run.

SPRING BARREL
The spring barrel contains the mainspring. It turns freely on an arbor, pulled along by the toothed wheel interacts with the first pinion of the movement's gear train. Some movements contain two or more spring  barrels for added power reserve.

STAFF or STEM

Arbor.

STOPWORK
Traditional device (now obsolete) provided with a finger piece fixed to the barrel arbor and a small wheel in the shape of a Maltese cross mounted on the barrel cover, limiting the extent to which the barrel can be wound.

STRIKING WORK
Sonnerie and Repeater.

SUBDIAL

Zone.

SUPER-LUMINOVA
Luminescent.

SWEEP SECOND HAND
A center second hand, i.e. a second hand mounted on the center of the main dial.