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Play three specific piano keys together, and you can instantly create the cheerful vibe of a pop anthem, the sorrow of a ballad, or the eerie tension of a horror movie soundtrack.
That three-note structure is a triad, built by stacking two intervals of a third on top of a starting pitch called the root.
How do three stacked notes create such completely different emotional moods?

The Building Blocks: Two Types of Thirds

An interval is the distance in pitch between two notes, measured in half steps (the distance between any key and the very next key on a piano keyboard).
Every triad is made of two basic ingredients: a major third (M3M3, measuring 4 half steps) and a minor third (m3m3, measuring 3 half steps).
Since we have two spots to fill and two flavors of thirds to pick from, what happens when we mix and match them?

The Four Triad Qualities

In 1722, French composer Jean-Philippe Rameau showed in his Treatise on Harmony that combining major and minor thirds produces exactly four fundamental chord types.
A major triad stacks a major third (4 hs4\text{ hs}) on the bottom and a minor third (3 hs3\text{ hs}) on top, creating a bright sound with a total span of a perfect fifth (7 hs7\text{ hs}).
A minor triad flips that order with a minor third (3 hs3\text{ hs}) on the bottom and a major third (4 hs4\text{ hs}) on top, giving it a darker, reflective sound while still spanning 7 half steps7\text{ half steps}.