Play only the five black keys of a piano in any order and it sounds GOOD — no wrong notes. That’s the pentatonic scale, and its secret is simple: it removes the two notes that could sit a semitone apart, so nothing ever grinds. It’s why it turns up in folk music on every continent and why a room full of strangers can improvise on it. Slide and hear the five safe steps.
The major pentatonic scale keeps five of the twelve chromatic notes — degrees {0, 2, 4, 7, 9} semitones from the root — and its defining property is that NO two of its notes are a semitone apart (the smallest interval is a whole tone). The semitone is the most dissonant small interval, and the tritone is absent too, so any combination of pentatonic notes sounds consonant, which is why it’s the universal ‘safe’ scale for improvisation and appears independently across world folk traditions. A fail-loud self-check throws unless the minimum interval between adjacent scale notes is at least two semitones. ◆ real music theory, node-verified.
The major pentatonic (its minor rotation shares the notes); the ‘always sounds good’ is about avoided dissonances, not a guarantee of interest — the no-semitone-clash property is exact and is why the scale is so forgiving.