---
title: "First Inversion — AP Music Theory Definition & Guide"
description: "First inversion puts the chord's third in the bass (figured bass ⁶). Learn how I⁶ expands tonic and how iv⁶ powers the Phrygian half cadence on the AP exam."
canonical: "https://fiveable.me/ap-music-theory/key-terms/first-inversion"
type: "key-term"
subject: "AP Music Theory"
unit: "Unit 5"
---

# First Inversion — AP Music Theory Definition & Guide

## Definition

First inversion is a chord voicing where the third of the chord is the lowest (bass) note, labeled with a ⁶ in figured bass (like I⁶ or iv⁶); in AP Music Theory it smooths bass lines, expands tonic, and defines cadences like the Phrygian half cadence (iv⁶-V).

## What It Is

A [triad](/ap-music-theory/unit-7/tonicization-through-secondary-dominant-chords/study-guide/rLCBduMA9EmMPYSSNiwd "fv-autolink") has three notes: [root](/ap-music-theory/key-terms/root "fv-autolink"), third, and fifth. Stack it normally and the root sits on the bottom (root position). Flip it so the **third is the bass note** and you've got first inversion. The chord's identity doesn't change. A C major chord with E in the bass is still a C major chord. What changes is the bass line, and in tonal music the bass line is doing a huge amount of the work.

In Roman numeral analysis, you mark first inversion with a superscript 6, so I⁶, vi⁶, iv⁶. That 6 comes from figured bass shorthand (the full figure is ⁶₃, meaning a sixth and a third above the bass). Composers reach for first inversion when they want the same harmony with a lighter sound or a smoother, more melodic bass line. That's why it shows up constantly in tonic expansions like I-vi-I⁶ and in [predominant](/ap-music-theory/unit-4/harmonic-progression-functional-harmony-cadences/study-guide/rGZpzBCL9qJX7z2WFbGx "fv-autolink") moves like iv⁶ heading to V.

## Why It Matters

First inversion lives in **[Unit 5](/ap-music-theory/unit-5 "fv-autolink"): Harmony and Voice Leading II**, supporting learning objectives **[AP Music Theory](/ap-music-theory "fv-autolink") 5.2.A** (identify and describe harmonic function and progression) and **AP Music Theory 5.5.A** (identify cadence types in performed and notated music). It matters for two big reasons. First, inversion is how you describe *the same chord doing a slightly different job*. I and I⁶ are both tonic, but I⁶ is weaker and more mobile, which is exactly why it works inside a tonic expansion like I-vi-I⁶ instead of at a final cadence. Second, the CED names one cadence that literally requires a first inversion chord. The **Phrygian half cadence** is iv⁶-V in minor (PIT-2.I.3), and if you can't spot the ⁶ on that iv chord, you can't correctly label the cadence. Inversion reading is also baked into every figured bass and harmonization FRQ, so this isn't optional vocabulary.

## Connections

### [Phrygian Half Cadence (Unit 5)](/ap-music-theory/key-terms/phrygian-half-cadence)

This cadence is defined by a first inversion chord. It's iv⁶ moving to V in minor, and the magic is in the bass, which walks down by [half step](/ap-music-theory/key-terms/half-step "fv-autolink") (scale degree 6 to 5). Without the inversion, you don't get that bass line, and you don't get the cadence.

### [Second Inversion (Unit 5)](/ap-music-theory/key-terms/second-inversion)

The next flip puts the fifth in the bass (⁶₄). The exam treats these very differently. First inversion chords are nearly as free as [root position](/ap-music-theory/key-terms/root-position "fv-autolink") chords, while second inversion chords are restricted to specific situations. Don't let the similar figures fool you.

### Predominant Function (Unit 5)

First inversion supercharges predominant chords by smoothing the bass approach to V. The iv⁶ in the [Phrygian half cadence](/ap-music-theory/key-terms/phrygian-half-cadence "fv-autolink") is the textbook case, and IV or ii⁶ in first inversion are everyday predominant choices in part writing.

### The vi Chord and Tonic Expansion (Unit 5)

Progressions like I-vi-I⁶ use first inversion to vary the tonic. Per PIT-2.J.1, vi can substitute for tonic, and ending the expansion on I⁶ instead of I keeps the phrase moving forward rather than sounding finished too early.

## On the AP Exam

Expect first inversion everywhere, not as its own question but as a skill embedded in others. Multiple-choice questions ask you to identify Roman numerals with correct inversion figures in notated excerpts, and aural questions can hinge on hearing that a bass note isn't the root. Practice questions in this unit use progressions like I-vi-I⁶ and ask you to explain the formal function of each chord, so you need to know that I⁶ is a weaker tonic that keeps a phrase going. On the FRQs, figured bass realization requires you to build first inversion chords correctly from a ⁶ figure, and the harmonization FRQ rewards you for choosing inversions that create a smooth, stepwise bass. And for cadence identification (AP Music Theory 5.5.A), the Phrygian half cadence only counts if that iv chord is in first inversion.

## First Inversion vs Second Inversion

First inversion puts the THIRD in the bass (figure: ⁶); second inversion puts the FIFTH in the bass (figure: ⁶₄). The practical difference is freedom. First inversion chords can go almost anywhere root position chords can, while second inversion chords are dissonant in context and only appear in a few sanctioned uses (like the cadential ⁶₄). If you write a random ⁶₄ chord where a ⁶ belongs in part writing, you lose points.

## Key Takeaways

- First inversion means the third of the chord is the lowest sounding note, and you label it with a superscript 6 (like I⁶ or iv⁶).
- Inverting a chord changes the bass note and the strength of the chord, but it does not change the chord's root or its Roman numeral identity.
- The Phrygian half cadence is iv⁶ to V in minor, and the first inversion iv chord is what creates its signature half-step bass descent.
- I⁶ is a weaker version of tonic, which makes it perfect for keeping a phrase moving inside a tonic expansion like I-vi-I⁶, but wrong for ending a strong cadence.
- First inversion chords are far more flexible than second inversion chords, which the exam restricts to specific contexts like the cadential ⁶₄.

## FAQs

### What is first inversion in music theory?

First inversion is a chord arrangement where the third of the chord is the bass (lowest) note instead of the root. It's labeled with a 6 in Roman numeral analysis, short for the figured bass ⁶₃, because the upper notes sit a sixth and a third above the bass.

### Does first inversion change what chord it is?

No. A C major chord with E in the bass is still a C major chord with the Roman numeral I in C major. Inversion changes the bass note, the sound, and the chord's strength, but never its root or function label.

### What's the difference between first inversion and second inversion?

First inversion has the third in the bass (figure ⁶); second inversion has the fifth in the bass (figure ⁶₄). First inversion chords are used freely, while second inversion chords are treated as dissonant and only appear in limited contexts, which matters a lot in AP part-writing FRQs.

### Why is the Phrygian half cadence written iv⁶ instead of just iv?

The first inversion is the whole point. Putting the third of iv in the bass means the bass moves down by half step into V (scale degree 6 to 5 in minor), and that distinctive half-step descent is what defines the Phrygian half cadence per the CED (PIT-2.I.3).

### Can a phrase end on a first inversion tonic chord?

It can in real music, but it won't sound conclusive. I⁶ is a weaker tonic than root position I, so authentic cadences that sound final use I in root position. On the exam, ending on I⁶ signals a weaker arrival or a phrase that keeps moving.

## Related Study Guides

- [5.5 Cadences and Predominant Function](/ap-music-theory/unit-5/cadences-predominant-function/study-guide/INNHEx3QCfTJPy2yXPqC)
- [5.2 The vi (VI) Chord](/ap-music-theory/unit-5/vi-chord/study-guide/PTZZPjhIlCfYCyREhl87)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-music-theory/key-terms/first-inversion#resource","name":"First Inversion — AP Music Theory Definition & Guide","url":"https://fiveable.me/ap-music-theory/key-terms/first-inversion","learningResourceType":"Concept explainer","educationalLevel":"AP / High School","about":{"@id":"https://fiveable.me/ap-music-theory/key-terms/first-inversion#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-12T22:52:45.590Z","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/first-inversion#term","name":"First Inversion","description":"First inversion is a chord voicing where the third of the chord is the lowest (bass) note, labeled with a ⁶ in figured bass (like I⁶ or iv⁶); in AP Music Theory it smooths bass lines, expands tonic, and defines cadences like the Phrygian half cadence (iv⁶-V).","url":"https://fiveable.me/ap-music-theory/key-terms/first-inversion","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Music Theory Key Terms","url":"https://fiveable.me/ap-music-theory/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is first inversion in music theory?","acceptedAnswer":{"@type":"Answer","text":"First inversion is a chord arrangement where the third of the chord is the bass (lowest) note instead of the root. It's labeled with a 6 in Roman numeral analysis, short for the figured bass ⁶₃, because the upper notes sit a sixth and a third above the bass."}},{"@type":"Question","name":"Does first inversion change what chord it is?","acceptedAnswer":{"@type":"Answer","text":"No. A C major chord with E in the bass is still a C major chord with the Roman numeral I in C major. Inversion changes the bass note, the sound, and the chord's strength, but never its root or function label."}},{"@type":"Question","name":"What's the difference between first inversion and second inversion?","acceptedAnswer":{"@type":"Answer","text":"First inversion has the third in the bass (figure ⁶); second inversion has the fifth in the bass (figure ⁶₄). First inversion chords are used freely, while second inversion chords are treated as dissonant and only appear in limited contexts, which matters a lot in AP part-writing FRQs."}},{"@type":"Question","name":"Why is the Phrygian half cadence written iv⁶ instead of just iv?","acceptedAnswer":{"@type":"Answer","text":"The first inversion is the whole point. Putting the third of iv in the bass means the bass moves down by half step into V (scale degree 6 to 5 in minor), and that distinctive half-step descent is what defines the Phrygian half cadence per the CED (PIT-2.I.3)."}},{"@type":"Question","name":"Can a phrase end on a first inversion tonic chord?","acceptedAnswer":{"@type":"Answer","text":"It can in real music, but it won't sound conclusive. I⁶ is a weaker tonic than root position I, so authentic cadences that sound final use I in root position. On the exam, ending on I⁶ signals a weaker arrival or a phrase that keeps moving."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"AP Music Theory","item":"https://fiveable.me/ap-music-theory"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/ap-music-theory/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 5","item":"https://fiveable.me/ap-music-theory/unit-5"},{"@type":"ListItem","position":4,"name":"First Inversion"}]}]}
```