eli5: music theory, particularly how changing the key so radically alters the sound of a piece.

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My middle school band class knowledge of music states that changing the key signature changes the number of sharps and flats in the piece. Fine.

But how exactly does this so radically alter a compositions?

When you switch a piece to a major or minor key it sounds completely different. Major keys are happy, minor keys are sad, as I understand it.

I really don’t understand what “happens” to a piece when it’s key signature is changed aside from the number of sharps and flats or why such a shift affects such a dramatic change.

I’ve seen videos of famous pieces put into different keys than we know them in and the change is both drastic and amusing. Any chance someone could help me understand this?

In: 1

Changing key generally doesn’t include completely changing from major to minor. It would typically be from maybe c major to b flat major.

Everything moves down two semitones. The overall piece sounds the same, just lower. All chord progressions from 1 to 4 are identical, as an example.

If you change to a minor key or vice versa then the chord progressions are no longer the same. The note progressions are no longer the same: c to e in major is 4 semitones but c to e (flat) in minor is 3 semitones. This changes the overall flow of music such that it sounds different.

Note that this may not really work for all pieces, composing in the minor key isn’t as simple as just pretending its major then flipping it. If you do that for most pieces it will probably not work well and sound both different and weird.

One reason you might want to transpose is for different instruments. A trumpet, for example, is typically sounded in b flat despite being written in c. If you try to play matching trumpet music on piano it won’t work unless you transpose. Alternatively, a singer might just have a deeper or higher voice than normal and needs the music put up or down to match.

a “key” is simply a collection of notes and associated chord progressions that those notes imply. Specifically in our Western European music language, a key is a 7 note set.

In most western music notation, there are twelve separate pitches. Each “key” only uses 7 of these twelve, and 5 can be considered “out of key”. This is why if you mess up and play an F instead an F# in a piece in G major, it’s going to sound a little funny.

Example: the key of G major is a 7 note set, with the first pitch starting on G. The rest of the pitches are: A, B, C, D, E and F#. If you are playing in this key, these will be the notes most frequently used.
Say all the sudden the piece shifts to B major though! Another 7 note set. Let’s tally the notes in that key: B, C#, D#, E, F#, G#, and A#.

How many notes are in common between those two scales? Only the B and E. Everything else is different (G# is played instead of G, C# is played instead of C etc.). The natural result of all these pitches being different is that the key of B major is going to “sound” very different when directly juxtaposed against G major.

If you want more details, head over to r/musictheory

The intervals between notes are different in major and minor keys. Major keys sound bright and happy. Minor keys sound moody and melancholy. Most pop and rock music is written in minor keys.