Introduction
A unison is two voices sounding the exact same pitch. It is the simplest interval — zero semitones — and the absolute reference point from which every other interval is measured. Unisons are not "boring"; they are the foundation of doubling, octave-doubling, and reinforcement in every kind of ensemble.
How to Find It on the Keyboard
Find any perfect unison in two simple steps. The number tells you the letter. The semitones tell you the accidental.
- Start on any root note. Count 1 letter name (including the root) up the musical alphabet — that gives you the top letter.
- Now count exactly 0 semitones from the root. If the natural top letter is too high or too low, sharpen or flatten it to land on the right pitch.
- Use the explorer above to check yourself in all 12 keys. The two highlighted notes are the P1 from that root.
Quick check: from C, the P1 lands on C. From G, it lands on G. From E♭, it lands on E♭.
Hear It — Song Associations for Ear Training
The fastest way to internalise the perfect unison is to associate it with a tune you already know. Sing the first two notes of any of these and you have the interval.
The Interval in Chords
Every chord is a stack of intervals. Here is where the perfect unison shows up in common harmony.
| Chord | Name | How P1 Appears |
|---|---|---|
| Doubling | Octave / unison doubling | Reinforcing a melodic line in two voices |
| Pedal | Pedal point | A repeated pitch under shifting harmony |
Inversion: Flip It Upside Down
When you move the bottom note up an octave (or the top note down an octave), the interval inverts. Two simple rules govern interval inversions:
- Numbers sum to 9. A 2nd inverts to a 7th, a 3rd inverts to a 6th, a 4th inverts to a 5th, and so on (1 + 8 = 9 for unison/octave).
- Quality flips. Major ↔ minor, augmented ↔ diminished, perfect stays perfect.
Compound Form
A compound interval is the same interval with an extra octave added on top. The character stays the same but the two notes are spread further apart. The compound form of the perfect unison is the Perfect Octave (P8) — 12 semitones in total.
Why it matters: chord extensions like 9ths, 11ths, and 13ths are compound intervals stacked above the basic triad. Move it up an octave and you get a wider, more open sound — common in piano voicings and orchestral spacing.
Enharmonic Equivalents
Two intervals are enharmonic when they sound the same but are spelled differently. Same physical pitches, different musical meaning.
- Diminished Second (d2) — same pitch, different spelling, e.g. C–D♭♭
On the keyboard, an enharmonic pair sounds identical. On paper, the spelling tells you which scale or chord the note belongs to — and that changes how it functions in the music.
Frequently Asked Questions
Is a unison really an interval?
Yes — formally a "perfect unison" is the interval of zero semitones. It is the reference against which every other interval is measured.
How does a unison sound?
It is the most consonant interval that exists — there is no acoustic beating because the two pitches are identical. The two voices simply reinforce each other.
What is the inversion of a unison?
The perfect octave. Numbers in inversion sum to 9 (1 + 8 = 9), and quality stays perfect.
What is a "diminished second"?
The same pitch as a unison spelled with two different letter names — e.g. C and D♭♭. Acoustically a unison; on paper a different interval.
Why does music use unisons?
For weight and emphasis. A melody doubled in unison sounds bigger; a unison riff in rock and metal hits harder than the same line played by a single instrument.