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victoria Williams Music Theory

Victoria Williams

LmusTCL BA Mus (Hons) MISM

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9. Method for Completing a Trio Sonata

Grade 8 Music Theory - Method for Completing a Trio Sonata

To make a good attempt at completing a Trio Sonata exam question, I’d suggest breaking the task down into four stages:

  1. Preparation
  2. Composing by numbers
  3. Free composition
  4. Checking


1. Preparation

Starting at the beginning, go through the piece looking at the figured bass. Underneath each change of chord, pencil in the following information:

  • Prevailing key
  • Letter name of chord (e.g. C major)
  • Roman numeral of chord (e.g. V)

You need to know the prevailing keys in order to work out the Roman numerals, and to make sure you never double the leading note.

You need to know the Roman numerals, so that you can understand the relationship of each chord to its neighbour – you need to know when a ii7 chord is being used, for example, so that you can approach the 7th correctly.

In addition, completing this task will give you a general understanding of how the piece sounds, more quickly than just trying to hear it in your head (but do try to hear it in your head too!)

Writing in the letter name of the chords will help you work out the Roman numerals, and will also make it quicker for you to see which notes should be included in each chord.

 method write chords

2. Composing by Numbers

A surprising amount of the sonata will “write itself”, if you follow the guidelines listed below. Completing the puzzle is a question of logic – what “can’t” be, and what “must” be!

Added 7th chords, 9ths and suspensions need to be handled with care – but actually this makes your job a little easier. Because you have to follow some rules to use them correctly, they take the guesswork out of what the melody should be doing in each part.

Use the guidelines to sketch as much of the outline “main melody” as you can.

  • No consecutive 5ths/8ves
  • Leading notes should rise to the tonic in V-I
  • Figured 7ths and 9ths cannot be omitted
  • Notes which must have been prepared in the same part in the previous chord:
    • 9ths in “9” chords
    • 7ths in ii7 chords
    • suspensions
  • 7ths in V7 chords must normally be either:
    • prepared or
    • approached by a leap
  • Suspensions and 7ths must resolve downwards by step
  • 3rds and roots cannot be omitted, with some exceptions:
    • Omit the 3rd in 9th chords
    • The 3rd may be omitted in diminished 7th chords
    • A final tonic chord can consist of three roots
  • Diminished triads should be complete (root, 3rd and 5th)
  • When part of a melodic figure has been started for you, often a sequence/imitation will fit.
    • Find the “main melody” notes in the original figure and see if they fit with the figured bass in another location.
    • If they do, copy over the melodic decoration too, to make the sequence/imitation
    • You may need to adapt a couple of notes of the original to make it fit the new harmony.
  • Avoid melodic augmented 4ths (not forbidden, but awkward) and leaps of a 7th (except as an approach to V7).

In this example, the main melody notes which must logically fit in the boxes are explained below.

method logical chords

  1. The A minor chord is missing its root, so this must be an A, above the previous G.
  2. The Bb major chord is missing its third, so this must be D.
  3. The ii7 chord has F as its 7th, which must be prepared in the previous chord, so this must be F (the preparation F is in bar 3).
  4. The Gm7 chord needs a third, so this must be Bb.
  5. The V7 chord has Bb as its 7th. The preparation is already made in [D], so this should also be Bb.

In this example, after the outline is sketched in, it becomes evident that the top part should be a sequence of the filled in middle part in the previous bar.

method outline


They should be decorated in the same way:

method decoration



3. Free Composition

Normally, you’ll be able to complete quite a large part of the sonata “by numbers”. You’ll then be left with a few completely blank beats or bars where you’ll need to invent something, and you will also need to add appropriate passing and auxiliary notes to keep up the rhythmic momentum.

This is, perhaps, the trickiest part of the task, because there is no one single correct answer.

Begin by filling in all of the “main melody” outline, and then add some melodic decoration to the whole piece. Here are some further tips to help you:

Copy ideas from elsewhere:

  • If you wrote a sequence, does the 2nd part also fit with the sequence?
  • Copy the same types of melodic decoration that are used elsewhere
  • Copy the same types of rhythmic groups that are used elsewhere
  • If rests have been used, add some in the same way


If two chord notes appear in the figure, but there is only space for one note, look either side to see if the other chord note features just before/after. If so, include the other one. Here for example, the figure asks for the 7 and 3#, both of which are missing.

method two figured notes

However, the bass has just played a D# on the previous beat, and would still be in the listener’s aural memory, and the 7th is more crucial, so the middle part should have A here.

If your “main melody” contains a repeated note, consider leaping upwards by an octave on the second note. This makes a more interesting melodic line, and is especially useful if you need to move into the correct part of stave to continue from the next given section.

Every time you add some melodic decoration, remember to check it against each of the other two parts for consecutive 5ths and octaves.

4. Checking Your Work

Aim to leave yourself enough time to check through your answer in the exam. While you are practising, you should also practise checking – the faster and more accurately you can do it, the better chance you will have on the day!


  1. Are the figures interpreted correctly (i.e. does the chord have the right notes in it)?
  2. Are all suspensions correct (preparation – suspension – resolution)?
  3. Are all 7th chords correct (preparation/approach – resolution)?
  4. Are all 9th chords correct (preparation)?
  5. Are accidentals added correctly (including cancellation when no longer needed)?
  6. Have you avoided consecutives on the main melody notes?
  7. Have you avoided consecutives within the melodic decoration?
  8. Do leading notes rise to the tonic?
  9. Are augmented intervals avoided/treated correctly?
  10. Have you avoided all three parts moving in similar motion (one beat is ok – no more!)?



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