Adolphe Sax invented the instrument that bears his name in 1838, and the saxophone has since become a familiar part of orchestras and bands in classical, jazz and popular music. Like many other wind instruments, the saxophone has its own key. The alto sax is in the key of E-flat, so it must make the appropriate adjustments to play music originally written for piano. To transpose piano music for the alto sax, observe some basic principles.
Make the key signature a major sixth above what is written in the piano music. A major sixth is five and a half tones: C to D (tone), D to E (tone), E to F (half-tone or semitone), F to G (tone), G to A (tone). For example, a piece in C major, with no sharps or flats becomes A major, with F-, C- and G-sharp. A piece in E-flat major, with B-, E- and A-flat, becomes C major, with no sharps or flats.
Play the notes a major sixth above what is written. Where the piano plays B, for example, play G-sharp, and where the piano plays G, play E.
Play notes a minor third below what is written, alternatively. A minor third is one and a half tones, as in E-flat to C: E-flat to D (half-tone or semitone), D to C (tone). This will sound an octave lower than the piano, but you may find it simpler to think in terms of a third below, rather than a sixth above. You may also think of it as transposing down a minor third and then up an octave, if you find the concept easier to understand.
If you choose to play a third lower, rather than a sixth higher, as part of a band or orchestra, you might have problems blending in with and being heard above the other instruments.