A Brief Guide to Species Counterpoint (cont.)
Third Species (four notes against one)
Third species requires four crotchets in each bar and dissonances may now appear on the second, third and fourth of these. Most of the same rules from earlier species apply including the prohibitions on leaping from dissonances and fourths. Parallel motion should still generally be avoided between the main beats.
[numbers refer to the interval between the parts - 6 = 6th etc.]
Note that in the last but one bar of the example all the rules are broken - in particular there is a leap away from a dissonance (the C on the second beat of the bar). This is called a nota cambiata and is traditionally allowed in third species counterpoint. Schenker usually devises rather complicated explanations of why an exception to the rules can be made for polemical reasons. His explanation in this case is, however, rather interesting, because it is a very simple example of an important principle of Schenkerian theory.
Schenker argues that the reason the nota cambiata is allowed is because it really consists of two allowable units (dissonances approached and left by step in the same direction) squashed together. The idea that a voice-leading progression (like that from E-D-C below) can be interrupted by another note (in this case B) and still retain the effect of the original progression is a crucial one. Schenker calls this addition of extra notes to a simple progression diminution - a term that we shall return to later.