Here’s a quick refresher. If you build a triad on each note of a major scale using only the notes in that scale, and you get seven specific chords whose qualities are completely determined by the scale. These are the diatonic chords of that key.

Let's build them in C major, since there are no sharps or flats to worry about. Start on each scale degree, skip a note, skip a note. Only use notes from the C major scale:

Check the quality of each one:

Scale degree	Chord	Quality
1	C - E - G	Major
2	D - F - A	Minor
3	E - G - B	Minor
4	F - A - C	Major
5	G - B - D	Major
6	A - C - E	Minor
7	B - D - F	Diminished

The pattern: Major, minor, minor, Major, Major, minor, diminished.

That pattern is the same in every major key. It doesn't matter whether you're in C, or G, or Eb, or F#. If you're in a major key, the chord built on the 1st scale degree is always major, the chord on the 2nd degree is always minor, and so on. The qualities are baked into the scale itself.

G major has one sharp: F#. Build the diatonic triads using only the notes of the G major scale (G A B C D E F#):

Scale degree	Chord	Quality
1	G - B - D	Major
2	A - C - E	Minor
3	B - D - F#	Minor
4	C - E - G	Major
5	D - F# - A	Major
6	E - G - B	Minor
7	F# - A - C	Diminished

Same pattern. Major, minor, minor, Major, Major, minor, diminished. Every time.

When somebody tells you a song is "in the key of D," they're not just telling you which notes to use. They're telling you which chords belong. Maybe that’s obvious, but it’s an important point that we want you to keep in mind.

This also explains why certain chords just seem to "go together." They go together because they all come from the same scale. They're a family.

• Two Minute Music Theory — Chords in Major and Minor Keys — very short explainer that covers the essentials.
• Piano From Scratch - Every Player Needs To Learn The Diatonic Chords of A Key — takes the concept a bit further with practical examples on the instrument.
• Piano With Jonny — Diatonic Chords: The Complete Guide — a thorough written guide with video and audio examples you can listen to.

Pick a key you're comfortable in on your instrument and build all seven diatonic triads. Say the chord names and qualities out loud as you play them. Then try a different key and do the same thing. The goal is to see the pattern repeating, not to memorize every chord in every key individually.

If you want to drill triad identification, the Note Trainer (chord mode) is a good way to build speed.