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What voice leading is
Voice leading is the art of moving from one chord to the next so that each individual line sounds smooth. A chord is not just a stack of notes: in good writing, each note belongs to a voice, like a singer in a choir, and each voice moves to a nearby note in the next chord. When voices move well, the progression sounds connected. When they jump around or collide, it sounds clumsy even if the chords themselves are correct.
The classic rules of voice leading come from choir writing, where four voices, soprano, alto, tenor and bass, sing together. Two rules matter most. First, avoid parallel fifths and parallel octaves: two voices moving together while keeping a perfect fifth or an octave between them. Second, avoid voice crossing, where a lower voice sings above a higher voice, which confuses the listener about which line is which.
Why parallels sound wrong
Parallel fifths and octaves are not wrong because of some ancient law. They sound wrong because the two voices fuse into one. A perfect fifth and an octave are such stable, hollow intervals that when two voices keep one while moving, the ear stops hearing two lines and hears a single thick line. The texture thins out, and the independence of the voices, which is the whole point of part writing, disappears.
This is also why the rule only applies to perfect intervals. Parallel thirds and sixths are everywhere in music, because those intervals keep the voices sounding separate. The rule targets exactly the intervals that make voices merge, and nothing else.
How to use the checker
Enter two chords as notes with octaves, listing the voices from lowest to highest, for example C3 E3 G3 C4 followed by D3 F sharp 3 A3 D4. The checker compares every pair of voices between the two chords and reports parallel fifths, parallel octaves, voice crossings, and spacing wider than an octave between neighbouring voices. If it reports nothing, the move is clean by the classic rules.
When the checker flags a parallel, fix it by changing one voice: move it in the opposite direction, or move it to a different octave of the same note. Often the simplest fix is to change the voicing of the second chord, for example by putting its third in a different voice, rather than changing the chords themselves.
Beyond the two big rules
Good voice leading is more than avoiding errors. Prefer small steps: voices that move by one or two semitones sound more connected than voices that leap. Keep common tones in the same voice when two chords share a note, since a held note is the smoothest move of all. And keep the spacing between neighbouring voices within an octave, except between bass and tenor, where wider spacing is normal because low notes need room.
These are guidelines, not laws. Pop, rock and jazz break the parallel rule constantly, especially on guitar, where parallel fifths are part of the sound. The checker tells you where the classic rules are broken so you can decide on purpose, not by accident.
Common mistakes
The most common mistake is entering the voices in the wrong order. The checker assumes your list runs from lowest to highest, so a scrambled list produces false alarms. Another mistake is forgetting octaves: without them, the checker cannot tell a fifth from a twelfth, and it cannot judge spacing. Always write notes with their octave numbers.
Finally, do not chase a perfect report at the cost of a good musical line. If fixing a parallel makes the melody worse, keep the melody. The rules serve the music, not the other way round, and a flagged parallel in a strong melody beats a clean report in a dull one.
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