The Complexity of Pianos
In the Harvard Dictionary of Music, 2nd Edition, the piano is introduced to the reader thus: "A stringed instrument whose strings are struck by hammers activated by keys, and which, since the end of the 18th century, has been the principle domestic keyboard instrument in Europe and America." A few lines later the entry goes on to say that the piano and the organ top all other instruments in complexity (the organ being ultimately, the most complex).
To give an example of the complexity of the piano, the article distinguishes the piano (originally, the pianoforte which means 'soft/loud') for its broad range -- "usually more than seven octaves.." -- then goes on to say:
The complexity of the piano arises from the fact that the soft felt hammer cannot merely be lifted toward the string like the tangent of the clavichord or carried past the string like the plucking mechanism of the harpsichord. Instead, the hammer must be thrown instead of being lifted, it must move faster than the key that activates it, requiring a lever system (called an action) between it and the key. (p. 671)
This system might seem simple enough, but the need for exactness is acute, especially if the pianist is to have exact control over the dynamics of the sound.
...the distance over which the hammer is thrown after the motion of the key is arrested must be kept small. On the other hand, the hammer must fall far enough away from the string after striking it so that there will be no possibility of its bouncing back up and accidentally restriking the string. (p. 671)
The article goes on to explain ways in which this complex mechanical movement is accomplished using a backcheck and an escapement, the lever that lifts the hammer is able to get out of the way just when the hammer hits the string. Because the lever is out of the way, the hammer "falls back further than it was thrown upward, even while the key is still held down."
Another aspect of the piano is that the hitting of the hammer upon the string can be done rapidly, even when playing the piano softly. This ability is made possible by an invention from the 19th century called "repetition action" or "double escapement" which allows the hammer to fall, but not all the way back to its resting position, where it can then be hit again and again before the hammer rests and the key returns to its untouched position on the keyboard.
These examples only scratch the surface of the incredibly complex masterpiece of human ingenuity that is the piano.