In music theory, a diatonic scale is a heptatonic scale that includes five whole steps and two half steps in each octave, in which the two half steps are separated from each other by either two or three whole steps, depending on their position in the scale. This pattern ensures that, in a diatonic scale spanning more than one octave, all the half steps are maximally separated from each other (i.e. separated by at least two whole steps). The word "diatonic" comes from the Greek διατονικός, meaning progressing through tones. The seven pitches of any diatonic scale can be obtained using a chain of six perfect fifths. For instance, the seven natural pitches that form the C-major scale can be obtained from a stack of perfect fifths starting from F: F—C—G—D—A—E—B Called Pythagorean tuning, this property of the diatonic scales was historically relevant and possibly contributed to their worldwide diffusion because for centuries it allowed musicians to tune musical instruments easily by ear. Any sequence of seven successive natural notes, such as C-D-E-F-G-A-B, and any transposition thereof, is a diatonic scale. Piano keyboards are designed to play natural notes, and hence diatonic scales, with their white keys. A diatonic scale can be also described as two tetrachords separated by a whole tone. The term diatonic originally referred to the diatonic genus, one of the three genera of the ancient Greeks. In musical set theory, Allen Forte classifies diatonic scales as set form 7–35. This article does not include alternative seven-note diatonic scales such as the harmonic minor or the melodic minor.

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