How can instruments be in a key?

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Background on me: I am a classically trained trombonist who struggles with music theory.

I know that a trombone is in the key of Bb, but what does that mean? The key is determined by the piece your playing? Additionally, a trombone with an F trigger is shifted into the key of F when the trigger is depressed (same with the G trigger on bass trombone). What does that mean? For me it just means that first position is now 6th.

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With instruments it means that the notation is conventionally transposed.

For example: a piano is easy to play in C and music is easy to read in C.

However, a Bb trumpet is easiest to play in Bb. So music for it is commonly written in Bb. But Bb is more difficult to read, unless you enjoy lots of accidentals in the key signature. So the music as written as though it is in C, but C on the stave is sounded as a Bb, D is sounded as a C, and so on.

It therefore means that music in the easiest to read key is easiest to play.

When a Bb instrument plays a C major scale (so no sharps or flats), it will be the same notes as a piano playing a Bb scale.

The length, thickness and maybe shape of the tube influence which notes are easiest to produce. So the size and shape is designed so that the easiest note to play might be Bb, and related notes like the relative fifth note and other harmonic notes are also easy to play just by changing the shape of your mouth or pressure of your breath.You can fine tune it by adjusting the mouth piece to make the tube a tiny bit longer or shorter. The slide makes the whole tube longer or shorter, allowing you to make a different set of notes.

To make it easier to read music, they make the notation for Bb look like C, so it will be easier to read.

Oops, repeating some of what was already posted.

Ok so for brass instruments, they used to not have valves. They basically had tuning slides which would changed the fundamental note of the instrument (for you this would be your pedal Bb). The music would be written the same, but the notes would be different. They would only play notes in the harmonic series (all the notes in first position on your trombone). This somewhat outdated convention remains. Horn in F was common, so french horn is written in F. Their F sounds like a concert C. This is different from your trombone being in Bb, your F sounds like an F, the Bb just describes the fundamental note on the instrument.

I’m a classically trained Pianist and Violinist. I also played Trumpet in band for a couple of years.

Sound as people hear it, is actually just vibrations in the air around us that make it into our ear canal and are processed by our brain.

In music theory, notes correspond to pitch frequencies in the audible range for humans. For example, “Middle C” on a piano is actually 262 Hertz. Hertz is a unit of measurement of frequency, and it just means that there were 262 “vibrations” that occurred within 1 sec.

If you have ever played in orchestra, and listened to the Violins while tuning, they tune to a 440 Hertz “A”.

So when you ask “what does it mean for a trombone to be in the key of B flat”? It means that the agreed upon frequency for standard trombone tuning is a 466.164 Hz “B flat”. When you play a note without adjusting the slide at all, it would be equivalent to walking up to a piano and playing a B flat. Same as if I blew into my trumpet without pressing any of the valves.

If something shifts a trombone into key of F, then I would assume without adjusting the slide at all, and only activating the shift, it would be the equivalent of walking up to a piano and playing an “F” (349.228 Hertz).

To further cause confusion, in band they had what was called “Concert” tuning, and IIRC, it would just transpose the band’s arrangements into the key of C in order to make sight reading easier/less accidentals in the score.

Some people are kind of missing the boat here. The key the music is written in is related to, but not exactly the same as the “key” of the instrument.

A B-flat trumpet, trombone, tuba, baritone/euphonium are said to be “in B-flat” because that’s the fundamental pitch of the instrument with no valves pressed/slide in first position. You’re a trombonist, so you know what I mean when I say “pedal B-flat,” right? That is the fundamental pitch produced by the length of tubing that makes up a trombone. A euphonium is the same length. A B-flat trumpet is half the length. A B-flat tuba is twice the length.

There are also C trumpets and C tubas often played by professional musicians…they can still play the same music written for B-flat tuba or trumpet on their C instrument…they just have to adjust their fingerings accordingly.

Now, for woodwinds…what makes it an “E-flat alto saxophone?” I’m not as sure of this. If you cover all of the holes, you get a concert-pitch D-flat. If you don’t put down any fingers, you get …E natural? I think that’s right.

Anyway, my best guess is that the E-flat scale is the most natural one to play by just putting fingers down and not using the “accidental” keys (like the pinky G-sharp key, the C-sharp key, etc). I am not totally sure of that. Just an educated guess.

In any case…what “key” the instrument is in, is a property of the *instrument*. The music is written transposed to that key for the sake of convenience and making it easier to move between instruments while keeping similar fingerings.

Just to add to this- people have given lots of great explanations about the physics of it. I want to add something about the way we notate them.
With any instrument, regardless of the “open” note mentioned in a different comment, we could write the given pitch as you would hear it. But in a lot of cases we choose not to for simplicity in reading notes. If your instrument range spends a lot of time above or below a standard clef then it becomes difficult to read with a million ledger lines.
Think of the notation system like panning in a video game (top down style like OG Pokémon or Zelda). If the screen never moved but most important stuff happened at the edges of the screen it would get annoying really fast. Writing the notes as pitches different than what you hear allows metaphorical panning. Even if your range is funny, you can read notes in the middle of the staff just as you are always the center of a screen when panning. Hopefully this makes sense!

r/musictheory is good for further info.

Carry on.

I’m not an expert and I have no idea how trombone works but I’ll give it a try:

>I know that a trombone is in the key of Bb, but what does that mean?

It means the instrument is capable of producing the notes in the Bb scale. Bb, C, D, Eb, F, G, and A.

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>The key is determined by the piece your playing?

Correct. Musics are written in a given key and if your instrument can’t produce the notes of that key you will need to transpose the song’s key to what your instrument can do.

A good example is harmonicas. Harmonica players usually carry around a case of harmonicas each one in a different key. If the song is in the key of C major they will use the Harmonica in C to play it. If the song is in the key of Bb and all they have is a harmonica in C major they won’t be able to play Bb and Eb(the instrument simply won’t produce those notes)

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> Additionally, a trombone with an F trigger is shifted into the key of F when the trigger is depressed (same with the G trigger on bass trombone). What does that mean?

I believe that’s just modifying the instrument’s pitch so now it can produce all the notes in the key of F.