What Is a Major Chord?

April 24th, 2022
In the discussion on my minimal chords post, someone commented:
I have no idea what any of this means (what is a chord? what is a major chord? what is a note? what is a first/fourth/fifth note? is there a 65th note? what is a scale? what is a major scale? what does it mean that a note is of a scale? what does it mean that a chord uses a note? is there a difference between a chord using a note of a scale and not of a scale?)

Here's an attempt to introduce enough music theory to answer these questions:

We hear changes in air pressure. If those changes are rapid enough and consistent enough, we hear them as pitch (frequency). We can talk about these in terms of how many changes we get per second, which we call "Hz". For example, a pitch could be 100Hz or 500Hz. When we say a pitch is "higher" or "above" another pitch, we mean more changes per second: 500Hz is higher than 100Hz.

A note is something that gives the impression of being a single pitch. For example, what you get when you play a single key on the piano, or pluck a string on a stringed instrument. Many instruments can only play one note at a time: trumpet, flute, saxophone.

The standard notes used in Western music differ in pitch by a factor of the 12th root of 2 (~1.06x). This means that if you go up twelve notes (which we call "half steps", confusingly) your pitch doubles (the 12th root of 2, multiplied by itself twelve times, is just 2). Two notes whose pitch differs by a factor of two (ex: 100Hz and 200Hz) are said to be an "octave" apart, and sound almost like the same note. We give notes that differ by some number of octaves the same name (ex: "C"), though when we want to be specific about which octave we're talking about we can append numbers ("C1" at ~32Hz is an octave above "C0" at ~16Hz).

A scale is a set of notes from an octave. We usually talk about a scale as being sorted from lowest note to highest. We can define a scale by the distances between its notes. Perhaps the simplest scale (the "chromatic scale") would be to go up by one note each time, playing every note: 111111111111. This typically doesn't sound very good, and we don't usually use it.

A "major scale" has the pattern 2212221: you go up by two notes, two notes, one note, etc. This gives you seven different notes in your octave. We can call these notes the "first", "second", etc notes of the major scale. We typically don't talk about "65th" notes because they would be way too high.

We name the notes with the letters A through G, which is only seven options for twelve notes. Each letter refers to a note that is one or two notes higher than the previous. For example, if we have the notes "A B C", to go from A to B we go up two notes, while from B to C we go up one note. To refer to the note we skipped when going from A to B we can say say "A#" ("A sharp") which means "start at A and go up one note" or "Bb" ("B flat") which means "start at B and go down one note". This is all very silly, but it's what we're stuck with for historical reasons. If you start with C and go up through the notes of the major scale, you will use the seven named notes: "C D E F G A B".

A chord is multiple notes played at the same time. The chords I was talking about in my post were "triads", which means they are three simultaneous notes. A major chord is notes one, three, and five of a major scale. A minor chord is the same, but the middle note (three) is moved down one note, which we call "flat" or "minor". You can also skip the third and play just notes one and five ("open fifths" or "power chords") which I do a lot on mandolin.

A key is the combination of a scale and a starting note. For example, "C major" is a major scale starting a C, while "D major" is the same but starting on a D. Most songs in traditional, pop, folk, and rock music draw all their notes from a single key, and all their chords will be built out of notes from that key as well.

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