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The Complete Musician by Steven Laitz states that melodic intervals larger than a fifth are considered dissonant and should be avoided. It also states that two leaps of a third are fine, provided you change direction afterwards, which Mozart does.

In Mozart's Sonata K545, the opening melody of the Allegro (one of the simplest and most popular melodies of the period) runs 1-3-5-7-8-9-8.

By the definition given in The Complete Musician, would the jump from the high 5 (G) down to the 7 (B) be considered a melodic dissonance, at it forms a minor 6th? This seems crazy, as the melody sounds extremely natural to me (and, I imagine, everybody else).

There are at least two explanations for why this leap is acceptable:

1. First is the idea of "gap fill," also sometimes called "registral return" or the "post-skip reversal." In short, when there is a large leap, we can soften it by subsequently moving by step in the opposite direction. This is a Gestalt principle of good melodic design that can often explain instances of "rule breaking."


2. Another concept is that the E and G in the first measure really just embellish the original C. As such, this isn't really a leap from G down to B, but rather just a simple step from the initial C down to B. This is a question of musical hierarchy: C is more important than the E or the G, so we therefore connect the larger-scale motion from C down to B, which is just a half step. Depending on what edition of the textbook you have, Laitz may address this concept in a section devoted to "compound melody."

It is traditional when teaching vocal writing to advise a preference for smaller intervals, which are easier to sing. A natural accent accrues to large intervals. The term melodic dissonance is sometimes used in connection to large intervals, the larger being more melodically dissonant.

By your book's definition, yes this is a melodically dissonant leap.

The advice to avoid such intervals is typical of advice to beginners not to break the rules until they're understood. Masters such as Mozart may do as they please, but if newcomers write melodies full of large leaps, they are likely to make an unsingable mess.

This usage should not be confused with consonant or dissonant harmony, nor with any idea of what sounds pleasant or unpleasant. It is a perhaps unfortunate piggy-backing of existing terms.

Could be that this rule in your textbook refers to the melody building of the Gregorian chant.

In the early church music till the time Palestrina there were some rules about intervals in a melody like a major sixth or bigger were considered as not good for singing.