Thirds, Fifths, and Sixths

After this lesson, you will be able to identify and play thirds, perfect fifths, and sixths as both melodic motion and harmonic sonorities. You will then place an intentional lower voice beneath a short four-note melody.

Thirds clarify major and minor color

Thirds span three letter names: C–E, D–F, E–G. In C major, C–E is M3, while D–F and E–G are m3. Played together, thirds often sound fuller than a single note while keeping two voices close. Played in sequence, a melodic third makes a clear contour without opening a very wide gap.

Do not assume every third is major. Count three letters, then check semitones or the notes of the key. The difference between four and three semitones becomes the foundation for distinguishing major and minor triads in the next lesson.

Fifths open the texture; sixths preserve two lines

P5 C–G has seven semitones and does not include the third that establishes major or minor quality. It can sound open and firm without identifying the chord's quality by itself. On piano, an open fifth can keep a bass texture clear. Avoiding parallel fifths is a rule in some strict part-writing styles, not a genre-neutral ban on moving fifths.

A sixth is the inversional partner of a third: invert C–E and it becomes E–C; M3 becomes m6. In piano textures, sixths create more space while leaving both voices easy to follow. Hear the upper pitch as melody and the lower pitch as support, not as a directionless block.

Choose by destination and voice motion

A melody note may accept several lower notes. Ask whether both notes fit the key, whether the interval creates the color you want, and whether the lower voice forms a playable line. You do not need a new interval beneath every melody note if that choice makes the lower voice jump without purpose.

In C major, C–E, D–F, and E–G create a parallel sequence of thirds. Changing the last pair to E–C creates m6: the melody rises to C5 while the lower voice holds E4. That retained note is a voice-leading choice, not the only answer.

Exercise

Four Notes on the Porch

Play the melody E4–F4–G4–C5 alone, holding each note for four beats. Next play C4–D4–E4–E4 as the lower voice. Combine them and keep the melody prominent. Name each interval with inclusive counting, then verify quality: C–E is M3; D–F and E–G are m3; E–C is m6.

Create a second version of measure 4 with P5 F4–C5. Compare E4–C5 with F4–C5. The first holds the lower E; the second continues upward by step. Either can work. Choose by desired line and sound, then explain your choice in one sentence.

Finally, play E4–F4–G4–C5 once without support and once with your lower voice. Notice how interval color changes the contour's character without obscuring the melody's direction.

Common mistakes

• Symptom: The lower voice matches or overpowers the melody. Correction: Practice each line alone, then combine with a clear upper voice and lighter support.
• Symptom: Calling every line-to-line or space-to-space pair a major third. Correction: After counting a third, verify three or four semitones.
• Symptom: Changing the lower interval every time and creating unnecessary jumps. Correction: Try a common tone or step before choosing a leap.

Frequently asked questions

Does a fifth always sound better than a third? No. P5 is open and lacks major/minor color; a third is fuller. Value depends on melody, bass, style, and register.

Why does M3 invert to m6? When the lower note moves up an octave, interval numbers add to nine and major becomes minor: C–E becomes E–C.

Does the four-note exercise have one correct answer? No. The written version has clear voice motion, while F4–C5 supplies another valid final measure.