There are three basic tunings for musical instruments, these being "concert" or C instruments (e.g. piano, guitar, flute or violin), B flat instruments (e.g. tenor sax, clarinet or trumpet) and E flat instruments (e.g. alto or baritone sax).
What this means in practical terms is that if you play the same written note on each of these three types of instruments, a different note will sound for each of the three types. The reason for the different tunings is the actual length of the tubing required to make the instruments themselves, along with ease of playing. The Bb and Eb instruments are referred to as transposing instruments. If you play a C note on a Bb instrument, the sound that actually comes out is a concert Bb note. If you play a C note on an Eb instrument, the sound that actually comes out is a concert Eb note. Once you have a good knowledge of intervals, it is easy to understand the relationship between the three tunings. If you see a note written for a concert instrument (e.g C) you would have to play a note one tone higher on a Bb instrument to get a concert C note to sound (play a D note). To sound a concert C note on an Eb instrument, you would need to play a note a major sixth higher (an A note). This is a general transposing principle with these instruments whenever you are playing with other musicians or are reading a part written for a concert instrument. Most sheet music is written for concert instruments, so transposing is an essential skill for any horn player to develop. If you are called in at short notice to play with a band, you will often be expected to transpose your part to the correct key immediately. If you can’t do this, they are likely to call another horn player who can!
Shown below is a C major scale written for concert pitch instruments, along with the correct transpositions for Bb instruments (up a tone – key of D) and Eb instruments (up a major 6th – key of A). As a result of these transpositions, all the notes come out sounding in the same key (concert key of C).