A B♭ trumpet reading a written C sounds a B♭. Pick a concert key or note and see what every transposing instrument has to read — or set a capo and find the key your chord shapes really sound in.
Concert to written: choose “Concert pitch”, pick the key of the song as it sounds, and read across: the alto sax column tells the saxophonist which key their part is in. Written to concert: choose “A written part”, pick the instrument and the note on their page, and the first row shows the note that actually sounds.
Capo: pick the shapes you are playing (say, G shapes) and the fret the capo is on. The table answers the reverse question: to sound in B♭ with G shapes, put the capo on fret 3.
Families of wind instruments are built in different sizes but share one fingering. Writing their music transposed lets a player switch from soprano to tenor saxophone without relearning which finger makes which note — the written C always uses the same fingers, and only the sounding pitch changes. Guitar and bass are written an octave higher than they sound so their music sits on the staff instead of on ledger lines.
To move a whole progression into a new key rather than one note, use the Chord Transposer.
What does a B♭ instrument read when the band plays in concert C?
It reads D — a whole tone higher. A B♭ tenor sax also reads D, but sounds an octave lower
than a B♭ trumpet.
What key does an alto sax read if the song is in concert E♭?
C. E♭ instruments read a major sixth above concert pitch, so concert E♭ becomes written C.
Does a capo change the chord shapes I play?
No. You keep playing the same shapes; the capo shortens every string by the same amount, so
everything sounds higher by one semitone per fret.