Many pipe organs have some or all of their pipes enclosed in swell boxes or chambers. Such enclosures are provided with openings for the egress of sound, over which are often fitted swell shades. Swell shades are arrays of pivoted boards, like large venetian blinds, disposed either horizontally or vertically, to control the egress of pipe sounds. In ancient times such shades were often simple doors, and not instantly adjustable by an organist, but in recent centuries organs have been fitted with pedals, called swell or expression pedals, whereby the organist can control the egress of sound at will. Many organs have multiple swell pedals controlling plural swell boxes or pipe chambers. Additionally, many organs are fitted with other similar pedals, called crescendo pedals, usually located to the right of the swell pedals, for engaging additional ranks of organ sounds when a more comprehensive ensemble of sound is desired. For about eighty years, musical organs without pipes have also been made, which also comprise expression pedals that are analogous to the swell pedals of pipe organs. In recent decades, advances in digital technology have made practical virtual pipe organs. Virtual pipe organs usually comprise a computer and operate by playing in loops digital files called samples having been recorded from sounds of pipes of real organ pipes. Virtual pipe organs also comprise swell pedals like those of pipe organs, which pedals control, either digitally or by analog circuitry, the levels of the signals emanating from virtual pipe chambers comprising ranks of samples of recorded pipe sounds. Expression pedals to control volume or to invoke other musical effects, such as sustain pedals for virtual pianos, are often attached to other musical instruments.