The piano belongs to the chordophone family according to the Hornbostel-Sachs system of classifying musical instruments. This system defines chordophones as instruments that produce sound through the vibration of strings that are stretched between two fixed points. The piano is classified under the subcategory of board zithers as an instrument where the strings are held by a board with a resonator box.
Erich Moritz von Hornbostel and Curt Sachs created the method of classifying musical instruments known as the Hornbostel-Sachs method. They first published it in 1914 in the German anthropology journal Zeitschrift fur Ethnologie. The system was based on an earlier method of classifying musical instruments devised by Victor Charles-Mahillon in the late 1800s.
Although the Hornbostel-Sachs system classifies the piano as a chordophone, it makes use of internal hammers that strike the strings, causing it to vibrate and produce the sound. To some degree, this mechanism also classifies the piano as a percussion instrument.
The internal hammers are each assigned to a particular note and are activated by pressing the piano keys that they are connected to. Once the hammer strikes the strings, the volume of the sound that is produced by the vibration is significantly increased by the resonator box that is also attached to the string frame board.