A cat piano or Katzenklavier (German) is a musical instrument with polyphonic aftertouch described by Athanasius Kircher. It consists of a line of cats fixed in place with their tails stretched out underneath a keyboard. Tails would be placed under the keys, causing the cats to cry out in pain when a key was pressed. The cats would be arranged according to the natural tone of their voices.

The instrument was described by German physician Johann Christian Reil (1759-1813) for the purpose of treating patients who had lost the ability to focus their attention. Reil believed that if they were forced to see and listen to this instrument, it would inevitably capture their attention and they would be cured (Richards, 1998).

from G. Schott, Magia Naturalis

This instrument was also described by the French writer Jean-Baptiste Weckerlin in his book Musiciana, extraits d’ouvrages rare ou bizarre (Musiciana, descriptions of rare or bizarre inventions):

When the King of Spain Felipe II was in Brussels in 1549 visiting his brother the Emperor Charles V, each saw the other rejoicing at the sight of a completely singular procession. At the head marched an enormous bull whose horns were burning, between which there was also a small devil. Behind the bull a young boy sewn into a bear skin ride on a horse whose ears and tail were cut off. Then came the archangel Saint Michael in bright clothing, and carrying a balance in his hand.

The most curious was on a chariot that carried the most singular music that can be imagined. It held a bear that played the organ; instead of pipes, there were sixteen cat heads each with its body confined; the tails were sticking out and were held to be played as the strings on a piano, if a key was pressed on the keyboard, the corresponding tail would be pulled hard, and it would produce each time a lamentable meow. The historian Juan Christoval Calvete, noted the cats were arranged properly to produce a succession of notes from the octave... (chromatically, I think).

This abominable orchestra arranged itself inside a theater where monkeys, wolves, deer and other animals danced to the sounds of this infernal music.