Online, I have researched what a piano is, and it says it is a percussion instrument. But when you look inside a piano, there are many strings, which are hit to give the piano its noise. Also, you can tune a piano - like any string instrument.
There are many instrument classification systems that have been used at different times and in different situations, so there is not one answer to your question. Here are probably the three most common classifications for the piano:
- Percussion due to the fact that the strings are struck by hammers. Other percussion instruments would include drums, tubular bells, and glockenspiels (the latter being a firmly pitched instrument). Organs and harpsichords would not be considered percussion in this system.
- Keyboard due to the fact that many keyboard instruments do not fit into other categories well, so some systems have a separate keyboard category. Other keyboard instruments in this system would include organs and harpsichords. Xylophones and glockenspiels would not be considered keyboard instruments in this system.
- Chordophone due to the fact that the sound is created by a vibrating string. Other chordophones include guitar and all members of the violin and viol families. A glockenspiel would be considered an idiophone since the sound is created by a vibrating piece of metal. An organ would be considered an aerophone since the sound is created by a vibrating column of air.
The "tuning" argument is irrelevant. Tuned drums are used in many different world musical traditions, as well as in western classical and popular music.
Instrument classification is to some extent arbitrary, and many different "systems" have been used in different cultures at different periods of history.
In western music, "keyboard instruments" are often grouped as a separate category, independent of how the sound is produced - plucking the strings (harpsichord), striking the strings (piano, clavichord), striking tuned percussion resonators (celesta), controlling a wind instrument (pipe organ, harmonium), or purely electronic sound generation.
It's both, and it's neither. Use such classifications only so far as they're useful to you. For instance, a piano strikes its strings, a harpsicord plucks them. So do we classify the harpsicord as closer to a violin, the piano as closer to a drum? And both as very different to each other? Only if it's useful to do so. Is it?
Since the strings inside a piano have to be hit by hammers, activated by pressing the keys, it is supposed to be in the percussion family. Other, earlier keyboards also had strings, but some were plucked somewhat like playing guitar, albeit the quills or equivqlent were activated by pressing the keys.So, although it has tunable strings, it's the method used to sound them that has the last word. Basically, anything that has to be hit - with anything!- is percussion.
The piano is a string based instrument that produces sound by pressing a key that is attached to a hammer. This hammer hits the string associated with the key. From this point of view the piano is more like a percussion instrument with the capability of tuning.
The predecessor of piano is the clavichord which is similar but more simple to the piano. It has no legs, it is placed on a table or any flat surfaces. Mostly has a rectangle shape. It was a very common instrument in the 15th century.
The other similar instrument is the harpsichord, but the main difference between them is that the strings are pulled or twanged instead of hit by hammers.