All of Chopin's compositions include the piano. Most are for solo piano, though he also wrote two piano concertos, a few chamber pieces, and some songs to Polish lyrics. His keyboard style is highly individual and often technically demanding; his own performances were noted for their nuance and sensitivity. Chopin invented the concept of instrumental ballade. His major piano works also include mazurkas, waltzes, nocturnes, polonaises, études, impromptus, scherzos, preludes and sonatas, some published only after his death. Influences on his compositional style include Polish folk music, the classical tradition of J. S. Bach, Mozart and Schubert, the music of all of whom he admired, as well as the Paris salons where he was a frequent guest. His innovations in style, musical form, and harmony, and his association of music with nationalism, were influential throughout and after the late Romantic period.

Answer this question, if possible (if impossible, reply "unanswerable"): Which instrument do every one of his compositions include?
piano