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Intervals are classified as consonant (stable, restful) or dissonant (tense, requiring resolution). Perfect unisons, fourths, fifths, and octaves, along with major and minor thirds and sixths are consonant. Seconds, sevenths, and tritones are dissonant. This classification is fundamental to voice leading and harmonic progression.
Listen carefully to pairs of consonant and dissonant intervals to internalize the sonic difference. Practice identifying them by ear as part of interval recognition training.
From your study of interval quality basics, you can already identify intervals by their size (seconds, thirds, fourths...) and quality (major, minor, perfect, augmented, diminished). Now we add a second axis of classification that is equally important for harmony: whether an interval sounds stable or unstable — consonant or dissonant.
The distinction is primarily perceptual. Play a major third (C–E) and then a minor second (C–D♭) on any instrument. The first settles; the second creates tension that seems to want to move somewhere. This difference in acoustic character underlies the entire tonal system. Tonal music is fundamentally a drama of tension and release, and consonance/dissonance is its mechanism: dissonances create the tension, resolutions to consonances release it.
The classification groups are worth memorizing in order of stability. Perfect consonances — unisons, octaves, and perfect fifths — are the most stable, sounding open and hollow, providing structural pillars in composition. Imperfect consonances — major and minor thirds, major and minor sixths — are slightly less stable but still restful; these intervals give chords their characteristic warmth. Dissonances include major and minor seconds, major and minor sevenths, and the tritone (augmented fourth/diminished fifth) — the most unstable interval in tonal music, spanning exactly half an octave. The perfect fourth is a borderline case: between two upper voices it sounds consonant, but above the bass in a chord it functions as a dissonance requiring resolution.
The practical application is voice leading. Because dissonant intervals create expectation, they must be resolved — typically by one or both voices moving by step to a consonant interval. A seventh resolves down by step; a tritone tends to resolve inward (both voices moving toward each other) or outward. This is not merely a convention but a feature of how listeners process harmonic tension: a dissonance sets up an expectation, and the resolution fulfills it. When composers deliberately leave dissonances unresolved — as 20th-century modernists did — the effect is unsettling precisely because that expectation is being denied.
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