A stabiliser or stabilizer (abbreviated to stab; also levelling mechanism in Cherry's terminology) is a device intended to stabilize a large keyboard key.
Stabilisers have two purposes:
- They prevent binding by helping to ensure that the keycap motion is vertical and that the keycap remains level no matter whereabouts it is pressed
- They prevent the play inherent in the switch slider from leading to significant wobble or tilting in larger keys, in particular space
Stabilisation is frequently applied to keys of 1.75 units width and greater. On a typical full-size US keyboard, this includes left and right shift, enter, backspace, and the plus, zero and enter keys on the numeric keypad.
The hardest key to stabilise is a big-ass enter key, and it's not unusual to have parts of this key be very difficult to press. The Zenith Supersport SX enter key has a complex stabiliser arrangement that is extremely difficult to assemble, but provides flawless transfer of force to the switch from anywhere across the keycap.
The scissor switch is actually a (typically plastic) stabilizer designed for all keys, not just large keys, on a low-profile keyboard, typically using rubber dome switches. Larger scissor switches can still be given extra stabilisation.
Many stabiliser designs have been used over the years. The common varieties are as follows:
The most common design is the wire stabiliser. A stiff wire runs from one end of the key to the other, connected to the keycap at each end, and to the keyboard plate or PCB via some form of restraint. The wire pivots against both the keyboard and keycap; pressing one end of the key rotates the wire, and the other end of the wire pulls its end of the keycap down accordingly. More than simply keeping the keycap moving vertically (important to prevent binding), this method actively keeps the keycap level.
This forms the basis of the stabiliser designs used by Costar, Cherry, SMK, and some of those from Alps and IBM.
NTC KB-6752EA internals, showing stabiliser wires
This design pairs a normal switch with a dummy switch. This approach has been used by Micro Switch for their Hall effect keyboards, as well as in ITW magnetic valve keyboards.
In the case of the ITW design, the stabiliser dummy switch feels quite light and seems to have low preload.
Some manufacturers such as Alps and Fujitsu used guide shafts for some or all large keys. A plastic insert is fitted, bearing a guide shaft, and a plastic post within the keycap fits into this shaft. This approach is not always successful; the Fujitsu FKB4700 is infamous for the poor quality of its stabilisation. However, this approach works well in IBM Model M keyboards.
Apple M0118 return key, with guide shaft stabiliser
Fujitsu FKB4700 enter key switch and guide shaft insert
The SMK-made "Type 2" keyboard for the Acorn BBC Microcomputer has been found with a pair of light weight switches under the return key. Matrix layout POS keyboards allow keycaps to span multiple switches in 1×2, 2×1 and 2×2 arrangements; here, the keys will be substantially heavier as a result.
IBM originally used wire stabilisers in the Model M keyboard, but changed to barrel inserts at the end of the 1980s.
Early Model M stabiliser
Cherry MX levelling mechanisms are intricate assemblies with keystems that allow for extremely easy removal and replacement of keycaps. The Cherry design also greatly reduces the noise associated with stabilisers. The chief complaint is that they dampen the switch feel, giving stabilised keys a stiffer and mushier feel. A small modification to the design—clipping a small piece off the sliders—does rectify this issue.
PCB-mount Cherry MX levelling mechanisms clip into the PCB. The PCB contains four holes, two per side; the rear stabiliser legs allow it to be swung into place via the rear holes, and prongs in the front place grip the front holes. The whole assembly can be inserted and removed easily.
Cherry levelling mechanism in a Cherry G80-3000
Levelling mechanism from a Cherry G80-3700
Cherry G80-3700 levelling mechanism mounted into PCB; one of the rear legs is not correctly inserted
Cherry ML keyboards use simple wire stabilisers.
Keyboards from Costar use simple wire stabilisers. Possibly as a result of the widespread adoption of Costar-made keyboards, all simple wire stabilisers get referred to as "Costar stabilisers".
Genuine Costar stabiliser in a Filco Majestouch
Non-Filco-style stabiliser in a Whitefox
The wires on these stabilisers bends outward—similar to the stabilisers provided with the WhiteFox and KC60—instead of perpendicular to the plate. 
Topre stabiliser for a function key on a Sony BKE-2011 as seen from the bottom of the keyboard with case and PCB removed.
Second-generation ITW magnetic valve keyboards used dummy switches for stabilisation. These switches lacked the ferrite core and magnet of live switches, and had a circular rather than square keycap mount.
SMK J-M0404 series uses slider lubrication to reduce binding on wider keys that do not have wire stabilisers. Apple describe these as "low friction" types.
Inserts are small plastic parts that connect the stabiliser wire to the keycap.
Costar stabiliser inserts from a Filco Majestouch 1