A perfect fifth (P5) is an interval spanning five letter names and exactly seven half steps, like C up to G. It is one of the most consonant intervals in music, which is why it anchors triads, defines key relationships, and gets special treatment in voice-leading rules.
A perfect fifth is an interval with a size of five (count the letter names: C-D-E-F-G is five) and a quality of perfect, meaning it contains exactly seven half steps. The shorthand is P5. Per the CED (PIT-1.L.1), every interval gets both a size and a quality, and the fifth is one of the intervals that can be perfect rather than major or minor. C to G, F to C, and G to D are all perfect fifths.
Watch the spelling, though. Size alone doesn't make a fifth perfect. B up to F is still a fifth by letter count, but it only contains six half steps, making it a diminished fifth (the tritone). Raise or lower one note with an accidental and the quality shifts. A fifth with eight half steps is augmented. The perfect fifth's stability is also why it shows up everywhere structurally. It's the distance between a triad's root and fifth, the gap between tonic and dominant, and the interval that separates each key on the circle of fifths.
The perfect fifth lives at the heart of Topic 2.5 (Interval Size and Quality) and learning objective AP Music Theory 2.5.A, which asks you to describe intervals in both performed and notated music. That means you need to recognize a P5 by ear and by eye. It builds directly on AP Music Theory 1.3.A from Unit 1, because quality is determined by counting half steps (PIT-1.C.1), and seven half steps is the number you're checking for. It also feeds AP Music Theory 2.10.A, since melodic transposition (PIT-3.C.6) requires keeping every interval intact, so if your original melody leaps a perfect fifth, your transposed version must too. Later, the perfect fifth becomes a voice-leading landmine: parallel perfect fifths between voices are forbidden in eighteenth-century part-writing, which is exactly what bass-line FRQs grade you on.
Keep studying AP Music Theory Unit 2
Visual cheatsheet
view galleryInterval Size and Quality (Unit 2)
The perfect fifth is the poster child for the size-plus-quality system in PIT-1.L.1. Five letter names gives you the size; seven half steps gives you the quality. Change either one and you have a different interval.
Half Steps and Whole Steps (Unit 1)
Half steps are the ruler you measure every interval with. A fifth is only perfect if it contains exactly seven of them, so the Unit 1 skill of counting semitones is what separates a P5 from a d5 or A5.
Melodic Transposition (Unit 2)
When you transpose a melody, every interval must survive the move (PIT-3.C.6). A leap of a perfect fifth in C major has to stay a perfect fifth in D major, which means rewriting it as seven half steps at the new pitch level, not just shifting noteheads.
Part-writing and Parallel Fifths (Units 4-6)
Eighteenth-century voice leading bans two voices from moving in parallel perfect fifths. The 2025 SAQ bass-line question tested exactly this, so spotting P5s between voices becomes a graded skill, not just a labeling exercise.
Interval identification shows up in both aural and written multiple-choice questions. You might hear two pitches and pick the interval, or see a notated pair and label it P5 (the shorthand itself is fair game). The trap answers are usually the diminished fifth (six half steps) and the perfect fourth (its inversion, five half steps), so count carefully. On the FRQ side, the 2025 SAQ Q7 asked for a bass line following eighteenth-century voice-leading procedures, where writing parallel perfect fifths between voices costs you points. The sight-singing and melodic dictation tasks also lean on the P5, since tonic-to-dominant leaps are among the most common melodic moves you'll need to hear and reproduce.
The perfect fourth and perfect fifth are inversions of each other. Flip a C-to-G fifth upside down and you get G-to-C, a fourth. The P4 spans five half steps, the P5 spans seven, and they add up to an octave. They're easy to mix up by ear because both sound open and hollow. One key difference for later units: the perfect fifth is always consonant, but a perfect fourth above the bass is treated as dissonant in tonal voice leading. A practice question asking when a perfect fourth is dissonant is really testing whether you know the fifth doesn't have that problem.
A perfect fifth spans five letter names and contains exactly seven half steps, written in shorthand as P5.
Letter-name distance alone isn't enough; B to F is a fifth by size but only six half steps, making it diminished, not perfect.
The perfect fifth is one of the most consonant intervals, which is why it sits between the root and fifth of every major and minor triad.
In melodic transposition, a perfect fifth must stay a perfect fifth at the new pitch level, or the melody is no longer the same tune.
In part-writing FRQs, two voices moving in parallel perfect fifths violate eighteenth-century voice-leading rules and lose points.
The perfect fifth inverts to a perfect fourth, and the two add up to an octave (7 + 5 = 12 half steps).
A perfect fifth is an interval spanning five letter names and seven half steps, like C up to G. Its shorthand is P5, and it's one of the most consonant intervals, forming the frame of every major and minor triad.
No. The fifth from B up to F contains only six half steps, making it a diminished fifth (the tritone). A fifth is only perfect when it contains exactly seven half steps, so always count semitones instead of trusting the letter names.
A perfect fifth is seven half steps and a perfect fourth is five; they're inversions of each other and add up to an octave. They sound similar by ear, but in voice leading the fifth is always consonant while a fourth above the bass is treated as dissonant.
P5. AP Music Theory labels intervals with a quality letter plus a size number, so a perfect fifth is P5, a diminished fifth is d5, and an augmented fifth is A5.
Eighteenth-century voice-leading style avoids two voices moving in parallel perfect fifths because the interval is so stable and hollow that the voices stop sounding independent. Bass-line FRQs, like the 2025 SAQ Q7, deduct points for them, so check every pair of consecutive chords.