In many cases there is a standard key for a song. It is thus written in C in the Standard Key. Thus the B flat book transposes it so that the Bflat instruments play it in the correct key. This does not mean you have to play it yourself in that key. Often differing groups of players stick to certain keys by default. Lots of folk style Canadian genre bands play in E and A. This is due to the fiddler being more comfortable with that.

That said, the book is not in Bflat. The book simply takes a piece normally played in the c book and transposes it for ease of use into the correct key for bflat instruments. That said, some bflat instrument players (ie Trombones) learn the positions as if playing in C thus need no transposition. Drives me nuts when figuring out which bone player needs which piece of music, Bass Clef C, Bass Clef Bflat, or which of the other two clefs and in which version .....

In the Practice (P) window just set band in a box to transpose your instrument for Bflat. So for example This is the End of a Beautiful Friendship is normally played in C by a Jazz Band. The transposition for Bflat puts the piece in D for a bflat horn. It is always up 2 semitones. So C goes to D. F goes to G. But if you do not like the key, say for example you are allergic to 5 flats do not play the piece in the key of B which for a horn transposes to D flat or 5 flats.

Playing or practicing alone the best bet is to set the software to Bflat by using the Practice (P) window, hit bflat, and then practice the piece in the keys you are most comfortable with. Set the global transposition to +2 and then transpose as usual in Band in a Box to whatever key you like.

John Conley
Musica est vita