O-ring

An o-ring is a small rubber ring fitted around the stem of a keycap, sitting just above where the keycap meets the switch housing, so that it cushions the moment the key bottoms out at the end of its travel. This softens both the sound and the physical shock of a full keystroke.

Because o-rings sit in the travel path, adding them also slightly reduces total key travel, which changes the feel of a keystroke in addition to quieting it. Thicker o-rings reduce travel more and dampen sound further, while thinner ones make a smaller difference, so the right thickness depends on how much change in feel a typist is willing to accept.

O-rings are a low-cost, reversible way to quiet a keyboard without opening up switches or stabilizers, which makes them popular for office use or shared spaces. A common downside is that they can make a switch feel mushy or inconsistent at the bottom of the stroke, which is why some enthusiasts prefer a mod that changes sound without shortening travel, such as case foam.