Clock Key

from Wikipedia, the free encyclopedia

A watch key (also watch key, French: Clef de Montre) is a handle element for tensioning the mainspring or winding the weight piece of the watch.[1]

In addition to the actual movement, a movement may also contain other subassemblies, such as an alarm movement, melody movement, or striking mechanism. These usually each have their own energy stores, such as hairsprings or drive weights.[2]

The tensioning of the mainspring and the winding of the weight are colloquially referred to as winding.

Old watch keys (about 1900-1960)

Modern watch keys (circa 1960-2000)

Double key for pendulums (Biedermeier period)

Pocket watch key (left around 1900, right around 2010)

Key pocket watch (around 1900)

Crank wrench for wall clocks with weight drive

Plug-in setting knobs for alarm clocks and small table clocks

Elevator key for a recorder (paper recorder, around 1960)

Elevator key for a punch clock (around 1970)


Socket wrench

Standard wrench with square

For winding spring-driven movements (wall clocks, table clocks), plug-in watch keys are common. As a positive connection between the winding stem and the socket wrench[3] and the socket wrench, the square has become established worldwide. This is the square-shaped end of a shaft or the spring core. The corresponding wrench has an inner square, which is only slightly larger than the square of the shaft.

Pendulum key

Some clocks, such as French pendulums, have a fine adjustment device with which the speed of the movement can be changed without touching the pendulum. For this purpose, there is a small opening with a square in the dial above the 12. By turning this square, the effective pendulum length is changed.

There are double spanners especially for such watches. At one end is the inner square for winding the mainspring, at the other end a small inner square for fine adjustment.

Pocket watch key

Before the development of the crown winding system, pocket watches had two square shafts. One was used to wind the mainspring, the other to set the hands. (see Indication (watch))

To protect the required pocket watch key from loss, the key was often attached directly to the watch chain.

Crank wrench

Crank wrenches are usually used for winding up the weight of regulators.

The weights of old tower clocks were originally wound with cranks. Tower clocks still in operation are now often equipped with an electric winding device.

Control knobs

Adjustable knobs are also variants of watch keys from a technical point of view, but are hardly ever referred to as such due to their shape and small size.
Adjustable knobs are mainly used on alarm clocks and small table clocks to set the alarm and time. Such adjusting knobs do not always have an inner square. To simplify production, many adjusting knobs have a round bore and are slotted at their clockwork end. This end sits clamped on the square-shaped end of a shaft. The slot enables a positive fit between the adjusting knob and the corners of the shaft square.

Special key

Pluggable keys for technical clock movements, such as paper tape recorders or punch clocks can have a wide variety of shapes
There are star-shaped universal wrenches especially for watchmakers. These have several pins with inner square edges in five different sizes. Universal wrenches with even and odd sizes are common (see table of wrenches for large watches)

Wrench for alarm clocks and small table clocks


To make watch keys captive, they and the winding stem are also threaded. Such wrenches or spanners[4] are mainly used on alarm clocks, travel clocks and table clocks.

If the watch is wound in the clockwise direction ( CW), a normal right-hand thread can be used. This is the case with most watches that are wound from the dial side.
In the case of movements which are wound from the back or which have a longer duration (e.g. 1 month), the winding is usually counter-clockwise (CCW). In this case, the watch key must have a left-hand thread.

The different movement assemblies may also have different winding directions.

Using a wrench also prevents damage to the movement if the wrench is turned in the wrong direction.

Whether the key has external or internal threads is not uniformly defined.


Due to the forces to be transmitted and for the purpose of long durability, watch keys are almost exclusively made of metals such as cast iron, zinc and brass. It was not until the middle of the 20th century that watch keys were also made entirely or partly of plastic.


Watch keys are cast in one piece from iron or zinc or assembled from individual parts by means of riveting, welding or brazing. If plastic is used, injection moulding is used.


At the beginning of watchmaking, each master craftsman worked according to his own ideas and experience. There were no standards. It was not until the transition to industrial watchmaking around 1900 that it became necessary to introduce uniform standards for types of gearing, threads and also watch keys.

Although watch key manufacturers and dealers agree on this gradation, virtually no reliable source of size gradation can be found. The following tables were therefore created using existing watch keys and dealer quotations.

Key for large clocks

In Germany, a graduation in steps of 0.25 mm (side length of the square) has become established for keys for large watches. However, watch keys are rarely marked with the size in millimeters. Mostly they carry a size number or no marking at all. The following table shows the sizes commonly used in Germany for large clock keys.

Size Square in mm
000 1,75
00 2,00
0 2,25
1 2,50
2 2,75
3 3,00
4 3,25
5 3,50
6 3,75
7 4,00
8 4,25
9 4,50
10 4,75
11 5,00
12 5,25
13 5,50
14 5,75
15 6,00

Keys for pocket watches

For pocket watch keys, a different gradation is applied in Germany.

Size Square in mm
00 2,00
0 1,90
1 1,80
2 1,75
3 1,65
4 1,60
5 1,50
6 1,40
7 1,30
8 1,20
9 1,15
10 1,05
11 1,00
12 0,95

Web links

Commons: Clock key– Collection of images, videos and audio files

Individual references

  1. Rudi Koch (ed.): “BI-Lexikon Uhren und Zeitmessung”, VEB Bibliografisches Institut Leipzig, 1989, 2nd revised edition, p. 227
  2. Rudi Koch (ed.): “BI-Lexikon Uhren und Zeitmessung”, VEB Bibliografisches Institut Leipzig, 1989, 2nd revised edition, p. 15-16
  3. Peter Held: “Uhrenbau Ein Werkbuch”, Historische Uhrenbücher Verlag: Florian Stern, Berlin, 2012, pp. 149-150
  4. Rudi Koch (ed.): “BI-Lexikon Uhren und Zeitmessung”, VEB Bibliografisches Institut Leipzig, 1989, 2nd revised edition, p. 227