A distinction is made in bipolar electrolytic capacitors between two construction types, "rough" and "smooth". "Rough" and "smooth" refer to the aluminium foil used for the winding of the capacitor. In the rough type the foil is roughened to increase the surface area of the foil. As the capacitance of a capacitor is proportional to the surface area, larger capacitances can be achieved in this way with the same material costs. This material saving is at the expense of slightly poorer electrical values. The construction size of rough electrolytic capacitors is significantly smaller. Thus, capacitances of up to 800 µF are available in the BR63 Series. Bipolar electrolytic capacitors are used where large capacitances are required cheaply (series resonant circuits, impedance linearisation, etc.) or where foil capacitors (M-CAP, MKT) are rejected for lack of space. The higher dielectric strength of the smooth typepermits their use where rough electrolytic capacitors are not surge-proof enough and more expensive foil capacitors are out of the question.