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Each instrument has a key. Trumpet is in B♭, alto saxophone is in E♭, etc. The key of the instrument has nothing to do with the notes you play. So, you gave the example of D and D♭. The ♭ next to the D means that you play the D a half step down. On trumpet, that will have a different fingering. So an E♭ is a half step lower than an E. An A♭ is a half step lower than an A. Does that make sense?

So, if on your test you see a D, just play a D. If you see a D with the ♭ symbol next to it, play a D♭ (however it says to on your fingering chart). Just play the notes you see. Don't flat a note unless you see the ♭ next to it. But, make sure to look at the key signature before playing. If there are flats or sharps in the key signature, all of those respective notes are flatted or sharped throughout the song unless otherwise noted.

Here's where it gets even more confusing: the other answerers are, in fact, correct. As I said earlier, each instrument has its own key. Trumpet's is B♭, as you know. But most other instruments have other keys, such as E♭. A trumpet's (key of B♭) C will be the same note as an alto saxophone's (key of E♭) G. So if a band director says to play a CONCERT B♭, which is a common tuning note, you would play a C on trumpet (a tenor sax would play a C as well), while an alto saxophonist would play a G. I know this is all super confusing right now, but things will get more clear as you improve and get used to it. Just keep practicing and you'll be able to see improvement and realize that reading notes and understanding all of this music language is getting easier.

For now, just play the notes you see on the page -- when playing music written for trumpet, you don't really have to worry about the fact that the trumpet is in the key of B♭, especially when you're a beginner.

Hope this helped a bit, and good luck!

DM is right, ignore the trumpet in Bb at the top of your page.

The Bb is useful for more advanced players or when you're in a full band (not just trumpet class). Your Bb is what we call a concert pitch.

If your director asked you to play a C, an alto saxophone to play a C, a french horn to play a C, and a tuba to play a C, it would sound terrible. All of the instruments I just mentioned have different sound properties, and your C won't sound like theirs. The concert pitch assigned to your instrument is what a tuba, baritone, flute, or trombone (all concert C instruments) has to play to sound like your C. It's useful when a director says "everyone play concert Bb." He's talking code to each player. Concert Bb will mean one thing for you, and another for each other instrument in the band. However, when everyone knows their code (everyone is transposing correctly), it will all sound the same.

It's confusing, but hopefully that helped a little.

Both the C trumpet and the Bb have the identical fingerings for the identical written pitch. However, the Bb trumpet sounds a entire step decrease than the written pitches. The gamers don't catch up on this reality; a written F at the first house continues to be performed with the primary valve irrespective of gambling a C or Bb horn.

One thing to keep in mind: Certain instruments are keyed in "concert key" meaning playing the note as written will sound as written. The trumpet is a "transposed" instrument: keyed in Bb. Playing the written note C will produce a Bb sound. The note you see on the staff will "sound" one whole note lower.