The capacitance of a component can be varied at will by adjusting the mutual surface area between the plates, or by changing the spacing between the plates. The two most common types of variable capacitors (besides varactors) are the air variable and the trimmer. You will also sometimes encounter coaxial capacitors.
By connecting two sets of metal plates so that they mesh, and by affixing one set to a rotatable shaft, a variable capacitor is made. The rotatable set of plates is called the rotor, and the fixed set is called the stator. This is the type of component you might have seen in older radio receivers, used to tune the frequency. Such capacitors are still used in transmitter output tuning networks. Following Figure is a functional drawing of an air-variable capacitor.
Air variables have maximum capacitance that depends on the number of plates in each set, and also on the spacing between the plates. Common maximum values are 50 to 500 pF; minimum values are a few picofarads. The voltage-handling capability depends on the spacing between the plates. Some air variables can handle many kilovolts.
Air variables are used primarily in RF applications. They are highly efficient, and are nonpolarized, although the rotor is usually connected to common ground (the chassis or circuit board).