Trimmer or Variable Capacitors are components used to adjust a portion of the electrical circuit or setting in a circuit that allows users to manually adjust capacitance. This makes them useful for tuning a circuit and "fine-tuning" its electrical characteristics. Trimmer capacitors are generally small and consist of two or more metallic plates of various shapes, each connected to one side of an insulated terminal that is placed in a fixed position such as a plastic shaft. The middle terminal is connected to an adjustment that can be increased or decreased. This adjustment changes the area of overlap between the plates, which in turn, adjusts the capacitance. In a simple circuit, trimmers or variable capacitors can be used to vary the frequency of an oscillator, the gain of an amplifier, or the impedance of a filter.