Piano duet

A piano duet is a piece of music written for two people to play at one piano. It is often called Piano 4 hands.
To play piano duets the two players sit with one person on the right (playing the high notes) and the other person on the left (playing the low notes). The person on the right is called "Primo" ("First") and the person on the left is called "Secondo" ("Second").
Piano duet music is usually printed with the Secondo part on the left-hand page and the Primo part on the right-hand page. Occasionally it is printed with the two parts underneath one another.
Playing piano duets is great fun and an excellent way to become a better musician. Like all group music making, each player has to learn to listen and adapt to what is going on in the other part. Sometimes one player will have to take away his or her hand quickly so that the other player can play the same note immediately afterwards. Sometimes the players have to cross their hands (secondo's right hand crosses with primo's left hand).
A lot of the time the primo's part may have both hands written in the treble clef and the secondo's part may have both hands written in the bass clef. This can sometimes be confusing at first.
The pedalling is usually done by the secondo player. This is because pedalling is very much about keeping chords going ("sustaining"), and it is the secondo who usually has the chords which give the harmony because these are in the lower part.
Repertoire
There are lots of piano duets available for people to play. Some of these are simple ones written for beginner pianists. Some are written by the great composers.
Some pieces that are really supposed to be played by an orchestra have been arranged for piano duet. In some cases the composer himself made this arrangement. Examples from famous composers include Stravinsky's Rite of Spring which is extremely hard to play, and Maurice Ravel's suite Ma Mere l'Oie.
Some very famous piano duets which were originally written in this form include several sonatas by Wolfgang Amadeus Mozart, several pieces by Franz Schubert including a Fantasia in f minor, Gabriel Faure's Dolly Suite, Debussy's Petite Suite and Georges Bizet's Jeux d'Enfants (which he also arranged for orchestra).
Piano Duos
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.
Piano Six Hands
There is also a small repertoire of pieces for three people to play at one piano. This is called Piano six hands. These pieces are often fun pieces which are not too hard to play, although there are examples of serious pieces. The three people need to be good friends, because it is a bit of a squash. The player at the bottom ought to do the pedalling but it may be easier for the middle player to do it.