The characteristics of resonant circuits, as just described, make them very useful for filtering various frequencies in an electronic circuit. Among the types of filters used in electronic circuits are high-pass filters, low-pass filters, and band-pass filters. A high-pass filter tends to pass frequencies in the higher ranges and to attenuate or reduce the current at frequencies in low ranges. The low-pass filter will pass frequencies in the lower ranges and attenuate or reduce the current frequencies of the higher ranges. A band-pass filter will allow a certain band of frequencies to pass and will reduce the current at frequencies below or above the band range. A filter may be made a tuning circuit by making either the inductance or the capacitance variable. A typical tuning circuit consists of a variable capacitor used with a fixed inductance. In some cases, however, the capacitor is fixed and the inductance is tuned by means of a "slug" or movable core. Tuning circuits are usually designed to have fairly high selectivity, that is, they allow only a very narrow band of frequencies to pass and reject all others.