If for example a tenor sax plays a C (position), you'll hear a Bb. So, if you write for tenor sax ( a Bb instrument), transpose the score up a major 2nd and all will sound fine.
practical example. A song is in C
- Piano reads in C
- Bass reads in C (sounds octave lower than written)
- Tenor sax reads in D (sounds major 2nd lower than written)
Quiz: Alto sax is an Eb instruments, so the score needs to be transposed by what interval? Up or down?