Building triads from a scale
Take a scale degree as the root, skip one scale note to get the third, then skip another to get the fifth. In C major, starting on D gives D–F–A: D minor.
The major-key pattern
The triad qualities in every major key are major, minor, minor, major, major, minor, diminished. In Roman numerals: I–ii–iii–IV–V–vi–vii°.
Uppercase numerals indicate major chords; lowercase numerals indicate minor chords. The degree symbol marks diminished.
Why the pattern is useful
Once memorised, it lets you work out the chords in any major key without memorising twelve separate lists. Find the scale notes, apply the quality pattern, then spell each chord correctly.
Natural minor
Natural minor produces i–ii°–III–iv–v–VI–VII. In tonal music, the fifth chord is often changed to major by raising scale degree seven, creating a stronger pull back to i.