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Cross-Relation in Music Theory: Definition & Role

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Wonder why your composition just doesn't sound right? Maybe it's got an unexpected cross-relation. In this lesson, we'll examine cross-relations and see how they should and should not be used.

Musical Relations

Sometimes you can just tell when a relationship isn't going to work. She's really outdoorsy, and he hates nature. He spends all his time volunteering at charities, and she's incredibly conceited. He is real, and she is an imaginary girlfriend in Canada. Those never work out.

It can be difficult enough for some people to manage a relationship between two independent voices, but musicians often have to do this with four. Of course, in this case we're not talking about dating; we're talking about polyphonic music, which contains multiple independent lines. There are a number of rules in polyphonic music, particularly four-part music, that provide guidelines on how to balance the relationship between the lines of music. The goal is to produce consonance, or full and harmonic sounds, rather than dissonance, the musical equivalent of nails on a chalkboard. One important rule in avoiding dissonance is to watch out for cross-relations. It's a musical relationship between two voices that rarely ends up working out.

Cross-Relations

Imagine writing a single musical line, and deciding to move the melody with chromatic alteration, or a movement of the same pitch by a half-step. For example, you could move from Bb to B, or F to F-sharp. There are reasons to do this, and if you do it correctly, it can sound pretty cool.

But what if this move happens not in a single line, but in two? While one voice moves to F, a different line moves to F-sharp, either at the same time or immediately after. When chromatic alteration occurs in a single voice, it can create a sense of chromatic movement, and you can do a lot with it. When this happens between two voices, there's no sense of clear movement. All you get is a clashing of voices and a lot of dissonance.

Cross-relation seen in the notes in red. Since the top line is two half notes, that eighth-note chromatic alteration is in a different voice and creates dissonance.
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We call this chromatic alteration in two voices a cross-relation, also called a false relation. In essence, it creates the auditory impression that one of your voices has suddenly dropped out of the key or shifted keys for a moment, while the other has not. So, it's generally something that composers are encouraged to avoid. To monitor for cross-relations in your compositions, watch out for accidentals (symbols that indicate the note is not a member of the scale like the sharp, flat, or natural signs). These can be good indicators of places where cross relations are likely to occur.

In four-part music, it can be easy to miss cross-relations if the two lines are in different clefs. Accidentals can help you spot this when it occurs.
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When to Use Cross-Relations

The general rule in polyphonic, and especially four-part, writing is to avoid cross-relations. They create dissonance and break up the motion between voices. However, composers have found that this effect can be exactly what they're looking for. In fact, cross-relations have been intentionally incorporated into polyphonic music and used by composers like Bach, Mozart, and Chopin ever since the Renaissance .

This is a tricky one- the cross-relation is occurring between two notes an octave apart, but that is still a chromatic alteration in two voices and creates the same sense of dissonance.
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