A capacitor is,
basically, two parallel metal plates separated by a small gap. Given a fixed gap and a
fixed surface area of the plates, higher frequency waves find it easier to 'jump the
gap' between the plates than lower frequency ones. Therefore, by varying the surface area of the plates,
the lowest frequency that can jump the gap can be set - only the higher frequencies being allowed to pass.

In practice, variable capacitors consist of many **pairs** of plates, one set being fixed
while the other set can be rotated into or out of mesh - thus altering the effective surface area.

The small picture to the left shows a pre-set type of capacitor in which the surface area is fixed but the gap between the plates can be adjusted by tightening a small screw. A smaller gap allowing lower frequecnies to pass.