The earliest keyboard music was written not for the piano, but for instruments like the organ, clavichord and harpsichord. Some of this music, though, is presently played on the piano and considered part of the piano's repertoire. Music specific to the piano began appearing in the mid-18th century and modern composers are still writing for the instrument today.
Music for the piano is broken down into several main types: Baroque, Classical, Romantic and 20th Century/Contemporary. The lines between them are occasionally blurred by pieces that were written during the transitional phases. Bach is a famous Baroque composer, while Beethoven is usually considered Classical. Liszt and Chopin are popular Romantic composers and Rachmaninoff is considered to be late-Romantic by some but 20th Century by others.
The Baroque period falls between 1600 and 1750, ending with the death of Bach. The Classical period lasted from 1750 to 1825. The Romantic period went from 1825 to 1900 and the 20th Century/Contemporary period began around 1900 and continues into the 21st century.
Most piano music was written in Western Europe and was concentrated in Germany, Austria, Italy, France and England. Russian composers came to prominence in the 19th century and Americans have also received some recognition, particularly in the 20th Century era. There are, of course, composers from many other countries, with the most notable previously mentioned.
The size of the repertoire has grown to great proportions over the years and includes music for solo piano as well as for piano duos, duets, trios, quartets, quintets and other arrangements.