some works are written for two people to play together using two pianos. these are usually called piano duos. examples include mozart’s sonata in d major k448. the sonata op 34 by johannes brahms started as a string quintet but he changed it to a piano concerto, then a piano duo, and eventually a piano quintet. the suite op 17 by rachmaninoff is another. most piano duos are very hard to play and need very good pianists. piano concertos can be played on two pianos, with one player playing the solo part and the other playing the orchestra’s music.