Some extremely old pianos have only 85 keys, ranging from A0 to A7. These pianos are no longer existent today. What's the reason?
Piano started with way less keys, the first models ranged on five octaves, being derived from harpsichords. Then in the Baroque period the keys increased to 85 keys. The shift to 88 keys happened because Steinway made an 88 key piano, and some composers, like Debussy, started to use it in the 20th century. The extra string also affects a bit the timbre of the piano because aren't dampen, so there's sympathetic resonance.