In 1722, Johann Sebastian Bach handed his eldest son a bound manuscript of 24 keyboard pieces - two in every possible major and minor key. He called it Das Wohltemperirte Clavier. What made it radical was not the difficulty. It was the argument embedded in every page: that all 24 keys were equally usable, equally expressive, and built on the same underlying structure. That structure was the triad - a three-note chord whose internal geometry Bach understood well enough to write a masterpiece in every key it could inhabit.
You are working with the same building block today.
A triad is three notes stacked in a specific pattern of thirds. A third is the distance of three letter names - C up to E, or E up to G. The bottom note is called the root. It gives the chord its name and acts as the gravitational centre everything else orbits. The middle note is the third. The top note is the fifth.
The fifth is mostly structural - it reinforces the root through acoustics you will feel before you understand. The third is where the personality lives.
Major and minor are a one-semitone difference
A semitone is the smallest step on a keyboard: one white key to the nearest black key, or one fret on a guitar. A major triad stacks four semitones between the root and the third, then three more semitones to reach the fifth. A minor triad reverses that: three semitones to the third, four more to the fifth.
That single swapped semitone - moving the third down by one step - is the entire difference between C major and C minor. Between a chord that sounds open and bright and one that sounds closed and heavy. Composers have made entire careers out of the emotional weight of that single note.
Think of it like a door hinge. The door itself (the outer frame: root to fifth) stays the same. The third is just the hinge position. Move it inward by one semitone and the whole door swings in a different direction.
Key Point: The third of a chord determines major or minor quality. The root and fifth stay the same between both versions of any chord. If you can find the third and know whether it sits three or four semitones above the root, you know the chord's character.
What the fifth actually does
The fifth sounds stable because of physics. When a string vibrates, it produces not just one frequency but a stack of them - the fundamental and a series of overtones above it. The fifth is the first non-octave note to appear in that overtone series, which means your ear already hears a hint of the fifth every time the root plays. When you add the actual fifth to the chord, you are reinforcing something that was already implied. That is why chords with the fifth removed sound thin: you have taken away the natural reinforcement your ear was expecting.
Guitar players who use power chords - just root and fifth, no third - end up with a sound that is loud and present but ambiguous in character. Strip the third and you strip the personality. The chord can sit under distortion without clashing because it has no quality to clash with.
Building the chord in any key
The formula does not change regardless of key. Major triad: four semitones, then three. Minor triad: three semitones, then four. Pick any note on any instrument, count up by those intervals, and you have the chord.
Start on D. Four semitones up lands on F#. Three more lands on A. D major. Start on D again. Three semitones lands on F. Four more lands on A. D minor. The letter names change; the architecture stays identical.
Bach wrote across all 24 keys because the structure scales perfectly. You can write in any of them for the same reason.