Electronic circuits and components
By keeping one plate (or set of plates) fixed and moving the other plate (or set of plates), a capacitor can be made variable. The dielectric material separating the plates can either be air or plastic (the latter tend to be more compact). Typical values for variable capacitors tend to range from about 25pF to 500pF. These components are commonly used for tuning radio receivers.
Ganged variable capacitors, in which two or three capacitors are linked together by a common control shaft are used to simultaneously tune several tuned circuits within a radio receiver.
Finally, you may also encounter various forms of preset capacitor that are used to make occasional adjustments (e.g. for precisely calibrating the frequency of an oscillator). Various types are available including solid-dielectric and air-spaced types.