Forbidden Parallels - Fifths and Octaves

Of all the rules in four-part writing, the best known and most strictly observed is the ban on parallel fifths and octaves.

Textbooks usually state the rule first. This lesson reverses that order: play it first, then work out why.

Listen first

With parallel fifths, and repaired

▶With parallel fifths▶Repaired

Check: Both move from V to I. In the first, tenor and bass rise together while holding a perfect 5th. Listen closely and those two voices stop sounding like two people and fuse into one thick tone. In the second the tenor goes somewhere else, and all four voices are audibly independent. The chord names are the same in both.

tenor and bass move in parallel, holding a perfect 5th

What you just heard is the whole of the rule.

Why they are forbidden

Four-part writing assumes four people singing independently. Each of the four voices has to be audible as its own melodic line.

Perfect fifths and perfect octaves are the most consonant intervals there are. As Unit 1 established, they fuse two notes into something that sounds like one.

So what happens when two voices move in the same direction while holding one of those intervals? They stay fused, and stop being distinguishable as separate lines. In effect the texture loses a voice.

Check all six pairs

This is where mistakes are most often missed. Do not stop at the outer voices.

Choosing two voices from four gives six combinations.

Pair	How easy to miss
soprano – bass	prominent, easy to notice
soprano – alto	easy to notice
soprano – tenor	far apart, easy to miss
alto – tenor	very easy to miss
alto – bass	easy to miss
tenor – bass	easy to miss

Parallels between inner voices are inconspicuous on the page and less obtrusive to the ear than outer-voice parallels. The rule treats them identically. Since there are too many pairs to track reliably by hand, this unit hands the job to the checker.

Check

Is checking soprano and bass sufficient to rule out parallel octaves?

No — all six pairs must be checkedChecking tenor and bass as well is sufficientOnly when the chord changesYes

Check answer

Contrary motion is treated differently

Two voices can move in opposite directions and still arrive at another perfect fifth. That is called consecutive fifths by contrary motion.

It is treated far less strictly than parallel fifths, because moving in opposite directions preserves the independence of the voices. Consecutive perfect consonances are still consecutive perfect consonances, so it is avoided where convenient.

The point is that prohibitions come in degrees of severity. Same direction is forbidden; opposite direction is a caution.

Hidden fifths and octaves

Separate from parallels, there is a motion called hidden (or direct) fifths and octaves.

Two conditions:

1. the outer voices move in the same direction into a perfect 5th (or octave)
2. and the soprano arrives by leap

The preceding interval does not have to be a fifth. The problem is that the arrival is conspicuous. A soprano sliding in by step goes unnoticed; a soprano landing by leap puts the perfect consonance in the spotlight.

This is treated as motion to watch rather than as a prohibition. It arises most often at cadences where the soprano leaps to the tonic, which is worth keeping in mind.

Voice crossing and overlap

Other rules protect voice independence too.

Voice crossing means an upper voice going below a lower one. If the alto drops beneath the tenor, a listener loses track of which line is which.

Voice overlap means a voice moving past the position a neighboring voice occupied immediately before. Nothing is crossed at any single moment, but in terms of motion one voice has overtaken the other. This is milder than crossing and is treated as a caution.

Watch for augmented intervals in minor

One last prohibition, this one on the melodic side.

The rule is do not move a melody by an augmented interval of a 2nd or larger. Minor keys are where it bites.

The qualifier “of a 2nd or larger” is deliberate. An augmented unison — a chromatic half step keeping the same letter, like A♭ to A — is outside this rule. What makes an augmented interval awkward to sing is leaping unnaturally wide across a short distance, whereas sliding a half step to the adjacent pitch is if anything smooth. In the chromatic harmony of Unit 8, that augmented unison becomes a basic move.

The C harmonic minor scale is C, D, E♭, F, G, A♭, B. The sixth degree A♭ and the leading tone B are adjacent, and the interval between them is an augmented 2nd.

Augmented 2nds are hard to sing and classical style avoids them. The remedy is fixed: lead the sixth degree down and the leading tone up. A♭ falls to G and B rises to C. Move them outward in opposite directions and the augmented 2nd never appears.

Check

Moving a melody from A♭ to B in C minor produces what interval?

minor 3rdaugmented 2ndperfect 4thmajor 2nd

Check answer

Write with the checker running

Listed all at once these rules are a lot to hold. Being corrected while actually writing makes them stick considerably faster.

▶ Look at a parallel-fifths example in the checker Load the parallel fifths example and play it, then change only the tenor note and watch the moment the warning disappears. Experiencing it vanish sticks far better than memorizing the rule SATB Part-Writing Checker →

The next lesson covers the procedure for connecting chords while avoiding these prohibitions — writing so that they never arise, rather than hunting for them afterward.