4.2.1 Inversion

[1] By definition, each voice in a complete four-voice triad has a 50% chance of being doubled. This is the case because doubling involves two voices, so any voice has two chances in four to be doubled. (By the same logic, if a PC is doubled in a two-voice texture, each voice has a 100% chance of being doubled.)

[2] As a result, specifying what inversion a triad is in constrains which doublings will occur. At first glance it might appear that each of the 3 PCs would have one chance in three of being doubled. Specifying an inversion, however, fixes one PC in the bottom voice. If the bottom voice has a 50% chance of being doubled, then so does its PC.

[3] Consider the specific example of a complete 4-voice root-position A-major triad. This triad has twelve possible configurations of PCs and voices, as shown in Figure 4.2.1a. Of those, there are six configurations that double the PC in the bass, but only three that double each of the other two PCs.

[4] To complicate matters, not all triad inversions are equally likely. Root position triads are the most common, and second-inversion triads are relatively rare. If triads are more likely to occur in root position, it follows that roots are correspondingly more likely to be doubled purely by chance.

[5] In order to compensate for this bias we paired the inversion of each random triad to that of its composed twin. By selecting a random triad only from the population of triads that have the same inversion as the composed triad, the random population has the same inversion bias as the composed triads.

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