A chordophone is any musical instrument who makes a sound by a vibrating string that is stretched between two points. Chordophones are one of the four divisions of the original Hornbostel-Sachs classification scheme. Hornbostel-Sachs divides the chordophone category into two main groups.
The first of the two groups is allocated with a classification number 31 and defined as “simple”. Instruments having a resonator is the second category having the number 32 defined as a “composite”. Most Western instruments fall into the second group. The Piano and Harpsichord fall into the first categories.
The idea is that the piano’s casing is considered its own resonator, in which it could be removed without destroying the instrument then it is classified as 31. It may seem odd, but if the action and strings of the piano were taken out of its box, it could still be played. This is not true of the violin, because the string passes over a bridge located on the resonator box. So removing the resonator would mean the strings had no tension.