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In music, a fourth is an interval spanning four staff positions in the musical notation common in Western culture.

Specific fourth intervals include:

• Perfect fourth, the fourth spanning five semitones
• Diminished fourth, a perfect fourth narrowed by a chromatic semitone, thus spanning four semitones
• Augmented fourth or tritone, an interval of three adjacent whole tones (six semitones)

In addition, fourth in music may refer to:

• Quartal harmony, harmonic structures built from the perfect fourth, the augmented fourth and the diminished fourth
• Subdominant, the fourth tonal degree of the diatonic scale