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Victoria Williams

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B4. Composition - Interpolation

Grade Six Music Theory Composition; Lesson 4 - Interpolation

Although it’s very common for musical phrases to be exactly balanced (e.g. four bars plus four bars), it’s certainly not always the case.

A sophisticated technique, which involves a padding out of the second phrase, is called interpolation. Instead of having two 2-bar sections in each phrase, the second phrase could contain three 2-bar sections, for example, resulting in a consequent which is six bars long: 


There are no hard and fast rules about interpolation sections – they can be different lengths and they are definitely not a compulsory part of the Grade Six music theory exam. However, you may get more points if you can use interpolation effectively! The instructions in the theory exam paper usually ask you to write a piece which is 8-10 bars in length, or at least eight bars long. A ten bar piece can be written with the structure 4+6, as shown above.

Interpolation is an effective technique because it is something which heightens our expectations and therefore increases the dramatic impact of the music. Our brain “expects” (subconsciously) each phrase to be balanced, so when the final phrase is extended in this way we become more alert to the music, waiting for the end to materialise. It creates a “wait for it!” moment. There is a feeling of tension or suspense, momentarily, which is only resolved when the final cadence is reached.



Our first example comes from Mozart’s Piano Sonata no.1, 3rd movement.




  • The antecedent is four bars long, and the consequent is six bars long.
  • Each phrase comprises two minims (half notes) followed by several semiquavers (16th notes).
  • The interpolation section begins in bar 7, although the listener will not be aware that there is an interpolation until bar 8. We are expecting semiquavers (16ths), because we heard them in the antecedent phrase in bar 3, and that’s what we get.
  • In bar 8, we would normally expect the phrase to end. This means we anticipate longer note values, most likely a minim (half note) or crotchets (quarter notes), and also probably a perfect cadence. Instead, we hear the exact same semiquaver (16th note) segment from bar 7, repeated. The cadence is delayed, which adds tension and suspense.
  • The perfect cadence and longer note values we expected by bar 8 appear in bars 9-10.


Our second example also comes from Mozart, and was chosen because it also illustrates that even antecedent phrases do not have to be balanced into two 2-bar sections. But notice that there still is balance and structure! This extract is 14 bars long – look at it carefully to see how 14 is achieved with balanced sections! This is Mozart’s Allegro in F, K15a.




  • The antecedent is 6 bars long, and the consequent is 8 bars long.
  • The antecedent contains three 2-bar sections. (Remember that the length of a section or phrase is calculated by the number of strong beats it contains.)
  • The consequent contains three 2-bar sections and a 2-bar interpolation.
  • In bar 11 we are expecting something similar to bar 5/section C, but instead we hear the rhythmic motif from section B again, as a melodic sequence. The end of the phrase is delayed.
  • In bar 13 we get the descending scale semiquaver (16th note) segment we were expecting in bar 11.




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