Everyone is acquainted with its imposing external form in the interior of churches. But is more difficult to define the instrument. What about this definition?: "The organ is a stationary instrument which is played with hands and feet on one or more keyboards, and in which the sound is produced by pipes (flue or reed pipes) arranged in ranks (voices or registers), which can be switched on individually by the player at will, placed over an apparatus for distributing wind (wind chest), this wind being provided by a special machine (bellows or blower)". After its "invention" by the Greek engineer Ktesibios (around 250 BC), the organ became established in the Hellenic and Roman Empires and, from there, throughout the whole of Europe. The decadent emperor Nero (54-68 AD) used it as plaything, and it was regarded as a symbol of entertainment and as the worldly showpiece of princes' courts until the early 14th century. It was only in the 15th century that this instrument, which had previously been condemned by the high dignitaries of the Church, started to be used for sacred functions as a kind of "machine for accompanying the choir" and took over the rule of the cult and church instrument par excellence. Different methods of construction and sound ideas developed in the various countries and regions, stemming mostly from the fancies of some prominent organ builder and his successors. With a zeal of almost Olympic proportions organs were constructed which were "bigger and bigger", "more and more beautiful", "more and more sonorous", in some of which it was impossible to use all the registers. In the largest church organ in the world, the five-part organ in Passau Cathedral, the organist could theoretically play on 231 registers with exactly 17,388 pipes, the biggest of which is more than 11 meters long and 47 centimeters in diameter, and the smallest only 6 millimeters long. U.K.