very cheap variable capacitors are constructed from layered aluminium and plastic foils that are variably pressed together using a screw. these so-called "squeezers" cannot provide a stable and reproducible capacitance, however. a variant of this structure that allows for linear movement of one set of plates to change the plate overlap area is also used and might be called a "slider". this has practical advantages for makeshift or home construction, and may be found in resonant-loop antennas or crystal radios.