--- title: "Perfect 5th — AP Music Theory Definition & Exam Guide" description: "A Perfect 5th spans five staff positions and seven half steps. It inverts to a Perfect 4th and is the transposition interval for Horn in F on the AP exam." canonical: "https://fiveable.me/ap-music-theory/key-terms/perfect-5th" type: "key-term" subject: "AP Music Theory" unit: "Unit 2" --- # Perfect 5th — AP Music Theory Definition & Exam Guide ## Definition A Perfect 5th is an interval spanning five staff positions (letter names) and seven half steps, such as C up to G. It's one of the most consonant intervals in Western music, it inverts to a Perfect 4th, and it's the transposition level for Horn in F on the AP Music Theory exam. ## What It Is A Perfect 5th covers five letter names and seven [half steps](/ap-music-theory/key-terms/half-step "fv-autolink"). Count C-D-E-F-G and you've got one, as long as the [half-step](/ap-music-theory/unit-2/chromatic-whole-tone-pentatonic/study-guide/cRVHFc77r9G0xbPnsvts "fv-autolink") count is exactly seven (C to G, yes; C to G♭, no, that's a diminished 5th). It's the interval from scale degree 1 up to scale degree 5 in both major and minor scales, which is why it sounds so stable and open. Power chords, the start of "Twinkle, Twinkle," and the root-to-fifth of every major and minor triad all live on this interval. The "perfect" label matters for how the interval behaves. Per the CED (PIT-1.M.1), [perfect intervals](/ap-music-theory/key-terms/perfect-intervals "fv-autolink") stay perfect when inverted, so a Perfect 5th flips into a Perfect 4th, and the two always add up to an octave (5 + 4 = 9). Stack an octave on top of a P5 and you get a Perfect 12th, its compound version. Same interval class, just spread wider. ## Why It Matters The Perfect 5th shows up in two [Unit 2](/ap-music-theory/unit-2 "fv-autolink") topics. In Topic 2.6 (LO 2.6.A), you identify inversions and compound intervals in performed and notated music, and the P5/P4 pair is the classic example of how perfect intervals invert into other perfect intervals. In Topic 2.7 (LO 2.7.A), the P5 becomes a practical tool because Horn in F sounds a Perfect 5th lower than written. Before you can analyze a score with a horn part, you have to mentally transpose every notated pitch down a P5 to find what actually sounds (PIT-1.N.1). Beyond Unit 2, the P5 is the backbone of triads, the [root](/ap-music-theory/key-terms/root "fv-autolink") motion of V to I cadences, and the spacing logic of the circle of fifths, so getting fast at spelling it pays off in every later unit. ## Connections ### [Perfect Fourth (Unit 2)](/ap-music-theory/key-terms/perfect-fourth) The Perfect 4th is the inversion of the Perfect 5th. Flip the lower note of a P5 up an [octave](/ap-music-theory/key-terms/octave "fv-autolink") and you get a P4, and their sizes sum to nine (5 + 4 = 9). The exam loves testing this pair because perfect intervals are the only quality that stays the same when inverted. ### Compound Intervals (Unit 2) A [Perfect](/ap-music-theory/unit-2/interval-size-quality/study-guide/HxrxB0vETN0eDp83zij1 "fv-autolink") 12th is just a Perfect 5th plus an octave. Exam questions hand you the compound interval and ask for the simple version (or vice versa), so the trick is subtracting 7 from the compound size. 12 minus 7 equals 5, and perfect stays perfect. ### Transposing Instruments (Unit 2) Horn in F sounds a Perfect 5th lower than notated. If a horn part shows C4-E4-G4, the sounding chord is F3-A3-C4, an F [major triad](/ap-music-theory/key-terms/major-triad "fv-autolink"). The P5 is the conversion key you apply to every single note in the part before analysis. ### [Consonance (Unit 1)](/ap-music-theory/key-terms/consonance) The Perfect 5th is among the most consonant intervals there is, which is why it anchors triads and cadences. When you label intervals by ear, that hollow, open, stable sound is your cue that you're hearing a P5 rather than something crunchier like a tritone. ## On the AP Exam Expect Perfect 5ths in interval identification (both aural and notated), inversion questions, compound interval questions, and especially transposition problems. A typical multiple-choice stem gives you a Horn in F part and asks for the sounding pitches or chord quality, which means transposing everything down a P5. For example, a notated C4-E4-G4 on Horn in F sounds as an F major triad. Another common stem gives you a Perfect 12th or Perfect 11th in a score and asks which simple interval is heard if the performer collapses the compound interval, where a P12 becomes a P5 and a P11 becomes a P4. The exam tells you each instrument's level and direction of transposition (per PIT-1.N.1), so you never have to memorize transpositions, but you do have to execute the P5 shift quickly and accurately, including keeping the accidentals right. No released FRQ names the Perfect 5th as a term, but melodic dictation and part-writing FRQs require you to hear and spell it constantly. ## Perfect 5th vs Perfect Fourth They're inversions of each other, so they share the same two pitch classes. C up to G is a P5 (seven half steps), but G up to C is a P4 (five half steps). The difference is which note is on the bottom and how far apart they are. If you mix them up on a transposition question, you'll shift the part by the wrong distance and every answer comes out wrong, so always count both letter names and half steps. ## Key Takeaways - A Perfect 5th spans five letter names and exactly seven half steps, like C up to G. - When inverted, a Perfect 5th becomes a Perfect 4th, because perfect intervals stay perfect and the sizes of an interval plus its inversion always sum to nine. - A Perfect 12th is the compound version of a Perfect 5th, meaning a P5 plus one octave. - Horn in F sounds a Perfect 5th lower than notated, so a written C4-E4-G4 horn triad actually sounds as F major (F3-A3-C4). - The exam provides the level and direction of transposition for each instrument, so your job is applying the P5 shift correctly, not memorizing it. - Five letter names alone doesn't guarantee a Perfect 5th. C to G♭ is a diminished 5th because it only has six half steps, so always check both size and quality. ## FAQs ### What is a Perfect 5th in music theory? It's an interval covering five staff positions and seven half steps, such as C to G or F to C. It's highly consonant and forms the root-to-fifth frame of major and minor triads. ### Is C to G♭ a Perfect 5th? No. C to G♭ spans five letter names but only six half steps, making it a diminished 5th. A true Perfect 5th needs exactly seven half steps, so C to G (natural) is the P5. ### What is the difference between a Perfect 5th and a Perfect 4th? They're inversions of each other. A P5 is seven half steps (C up to G) while a P4 is five half steps (G up to C). Same two notes, different note on the bottom, and their sizes add up to nine. ### What does a Perfect 5th invert to? A Perfect 4th. Per the CED, perfect intervals remain perfect when inverted, and an interval plus its inversion always equals a perfect octave. ### Why does a Perfect 5th matter for Horn in F on the AP exam? Horn in F sounds a Perfect 5th lower than written, so you must transpose every notated pitch down a P5 before analyzing the part. A notated C4-E4-G4 horn triad actually sounds as an F major triad. ## Related Study Guides - [2.7 Transposing Instruments](/ap-music-theory/unit-2/transposing-instruments/study-guide/8R8xuurFXzvX6HCFklDp) - [2.6 Interval Inversion and Compound Intervals](/ap-music-theory/unit-2/interval-inversion-compound-intervals/study-guide/fSSC2N938I34CjcsI5m5) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-music-theory/key-terms/perfect-5th#resource","name":"Perfect 5th — AP Music Theory Definition & Exam Guide","url":"https://fiveable.me/ap-music-theory/key-terms/perfect-5th","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-music-theory/key-terms/perfect-5th#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-12T23:22:06.581Z","isPartOf":{"@type":"Collection","name":"AP Music Theory Key Terms","url":"https://fiveable.me/ap-music-theory/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-music-theory/key-terms/perfect-5th#term","name":"Perfect 5th","description":"A Perfect 5th is an interval spanning five staff positions (letter names) and seven half steps, such as C up to G. 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