Second Inversion Triad

A second inversion triad is a triad with its fifth in the bass (lowest voice), labeled with the figured-bass symbol 6/4. Because the bass forms a fourth with the root above it, the chord sounds unstable, so tonal music treats it carefully rather than as a freestanding harmony.

What is Second Inversion Triad?

A triad is built from three pitches stacked in thirds, called the root, third, and fifth. Inversion is all about which of those three notes sits in the bass. Root position puts the root on the bottom, first inversion puts the third on the bottom, and second inversion puts the fifth on the bottom. The root and third can appear in any order above the bass; only the lowest note determines the inversion.

In Roman numeral analysis, second inversion gets the figured-bass label 6/4, because the upper notes sit a sixth and a fourth above the bass. So a tonic triad in C major (C-E-G) with G in the bass is written I6/4. Here's the catch that makes 6/4 chords special. That fourth above the bass was historically treated as a dissonance, so second inversion triads sound unstable and want to resolve. That instability is exactly why composers use them in specific, predictable situations (like the cadential 6/4 leading into a dominant chord) instead of dropping them anywhere in a progression.

Why Second Inversion Triad matters in AP Music Theory

Second inversion triads live in Unit 3: Music Fundamentals III - Triads and Seventh Chords, under Topic 3.1, where learning objective 3.1.A asks you to describe the quality of a chord in both performed and notated music. Inversion doesn't change a chord's quality (a major triad is still major upside down), but you have to see past the inversion to identify the quality at all. If you see G-C-E and try to stack thirds from the bass, you'll misname the chord. Recognizing that it's a C major triad in second inversion is the skill.

This term also sets up the harmony and voice-leading units later in the course, where 6/4 chords get their own rules. The cadential 6/4, passing 6/4, and pedal 6/4 all depend on you instantly recognizing second inversion in notation and by ear. Nail the 6/4 label now and the part-writing FRQs later get much easier.

How Second Inversion Triad connects across the course

Inversion (Unit 3)

Second inversion is one of three bass positions a triad can take. The whole system runs on one question. Which chord member is the lowest note? Fifth in the bass means second inversion, every time, no matter how the upper voices are arranged.

Chord Quality (Unit 3)

Inversion and quality are independent. A C major triad in second inversion is still major. On the exam, you often have to mentally re-stack an inverted chord into thirds before you can call it major, minor, diminished, or augmented.

Triad (Unit 3)

Everything here builds on the triad's three members. The fifth is normally the most stable interval above the root, but flip it into the bass and it creates a fourth against the root, which is what gives 6/4 chords their unsettled sound.

Diminished triad (Unit 3)

Quality identification gets trickier in inversion. A diminished triad in second inversion hides its signature tritone between the bass and an upper voice, so spelling the chord back into root position is the safest way to confirm its quality.

Is Second Inversion Triad on the AP Music Theory exam?

Multiple-choice questions test this skill from both directions. You might see a notated chord like G-E-C and need to identify it as a C major triad in second inversion, or hear a chord performed (often arpeggiated) and identify its quality and bass position. Roman numeral questions expect you to attach the 6/4 figure correctly. No released FRQ asks you to define the term, but the part-writing and harmonization FRQs in later units assume you can read and write 6/4 symbols fluently, and the figured-bass realization task uses these numbers directly. The fastest exam move is mechanical. Find the bass note, stack the chord in thirds on scratch paper, and check where the bass note falls in that stack.

Second Inversion Triad vs First inversion triad

First inversion puts the third in the bass and is labeled 6 (short for 6/3). Second inversion puts the fifth in the bass and is labeled 6/4. The practical difference is bigger than one note. First inversion chords are stable and used freely in progressions, while second inversion chords are unstable (because of the fourth above the bass) and appear only in controlled contexts like the cadential 6/4. If you're unsure which inversion you're looking at, measure the interval from the bass up to the root. A sixth means first inversion; a fourth means second inversion.

Key things to remember about Second Inversion Triad

• A second inversion triad has the fifth of the chord in the bass, and only the bass note matters, since the root and third can be arranged in any order above it.

• The figured-bass label for second inversion is 6/4, because the upper chord members sit a sixth and a fourth above the bass.

• Inversion never changes chord quality, so a major triad in second inversion is still a major triad for the purposes of LO 3.1.A.

• Second inversion triads sound unstable because the bass forms a fourth with the root above it, which is why tonal music limits them to specific uses like the cadential 6/4.

• To identify any inverted chord quickly, restack the notes in thirds to find the root, name the quality, then check which member is in the bass.

Frequently asked questions about Second Inversion Triad

What is a second inversion triad in music theory?

It's a triad voiced with its fifth as the lowest note. In C major, that means a C-E-G triad with G in the bass, labeled I6/4 in Roman numeral analysis.

Does the root have to be on top in a second inversion triad?

No. Only the bass note determines inversion. As long as the fifth is the lowest note, the root and third can sit in any order above it and the chord is still in second inversion.

What's the difference between first and second inversion?

First inversion has the third in the bass and is labeled 6; second inversion has the fifth in the bass and is labeled 6/4. Quick check: the interval from the bass up to the root is a sixth in first inversion but a fourth in second inversion.

Why is a second inversion triad called a 6/4 chord?

Figured bass numbers measure intervals above the bass. With the fifth in the bass, the root sits a fourth above and the third sits a sixth above, giving the figures 6/4.

Can you use a second inversion triad anywhere in a chord progression?

Not in common-practice style. The fourth above the bass makes the chord unstable, so 6/4 chords are reserved for specific patterns like the cadential 6/4 resolving to V, plus passing and pedal uses you'll study in the voice-leading units.