A common area of confusion for students revolves around non-transposing and transposing instruments. I wanted to write a quick post on the topic to clear things up. First, we have to understand what non-transposing and transposing instruments are. In the case of non-transposing instruments, the notes written on the page are the actual notes being sounded. However, in the case of transposing instruments, the notes written on the page differ from those that are sounded. Suppose we look at concert pitch which centers on the A above middle C, considered A=440Hz. If that A is written for the piano, and the pianist plays it, you hear that exact note A=440Hz.
On the other hand, if a transposing instrument such as a B flat trumpet plays that same A above C, while the note may be written as A=440Hz, the sound you are actually hearing is a major second below what is written; you actually hear a G. So in transposing instruments, they transpose, the note that is written for them is different from that which is sounded.
So to keep the orchestra in tune, you must transpose all the transposing instruments. For example, if I wanted a piano and B flat trumpet to play the same C, I would have to write a D for the trumpet because it sounds a major second below. A major second below D is C, so both instruments now would be playing the same note.
I’m not sure this post has been much help, but hopefully, it has cleared up some confusion regarding non-transposing and transposing instruments. No wonder this topic has become so confusing. Because what you see on the page, if you play a transposing instrument, is not what is being sounded. This may be a good topic for a video explanation. Thanks for reading; I’m just a music teacher having fun; catch ya on the next one.