The passing tone

A dissonance may connect two consonances by step, entering and leaving in the same direction, provided it falls on the weak part of the beat or bar: the passing tone. It is the mildest dissonance because it is pure motion — the voice never stands on the dissonant tone but moves through it, and the ear hears the line, not the clash.

The passing tone turns a bare succession of chord tones into a line: whenever two consonances lie a third apart, filling the gap with a single step is the standard way to connect them, and second species is built on it. It is available on any weak beat regardless of the harmony elsewhere, which makes it the one dissonance reached for almost without asking permission — no preparation, no obligation beyond continuing the direction already in motion. Because the ear tracks it as motion, it is the safest way to add activity to a static line.

• Placing the passing tone on the strong part of the beat instead of the weak, which turns it into an accented dissonance that needs its own separate justification.
• Approaching or leaving it by leap rather than by step, which abandons the stepwise connection that makes it a passing tone rather than some other figure.
• Reversing direction at the passing tone — stepping up into it and leaping back down, say — when the whole point is to keep moving the same way in and out.
• Doubling the passing tone in another voice, which fixes the ear’s attention on a tone that is supposed to be heard only in transit.
• Letting the passing tone create parallel fifths or octaves with another voice, the same forbidden parallels dressed up as incidental because the offending tone is a dissonance.

• Johann Joseph Fux, Gradus ad Parnassum (1725), Liber secundus, second species (the passing dissonance) — The species that admits dissonance for the first time — only in passing, only on the weak half.
• Johann Philipp Kirnberger, Die Kunst des reinen Satzes in der Musik (1771), Vol. I, the treatment of dissonance (incidental dissonances) — Kirnberger’s incidental dissonance: the passing tone needs no preparation because it displaces nothing.