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C5. Key

Grade 6 Music Theory lesson C5: Key

Key and Modulation

The subject of key is discussed in depth in the grade 6 composition course.

In particular, we looked at how to determine what key the start of a melody is in. We saw that the primary chords, I and V (or i and V in a minor key) are the most important chords because they help to fix the key.

When you are presented with a score in the grade six music theory exam, you might be asked to determine the key at any point in the score. There are three basic ideas to bear in mind: 

  • The key that the piece starts in is its main key. The piece is “in” that key. The piece will end in the same key.
  • The music might modulate to another key. A modulation is a key change which is substantial, settled and lasting. It lasts for several bars and may or may not be accompanied by an actual change of key signature. Modulations often happen when the mood of a piece changes, or a new section is started. With a modulation, a new tonic is established.
    When a modulation occurs without an actual change of key signature, we can describe the new key as the “prevailing key”.
  • The music might pass through any number of keys. If a key is “passed through”, it is only touched upon for a very short time – just a few bars or less, for example. It is not “settled” – we don’t get the feeling that a new tonic has been established. Instead, we feel that the key of the music is in transition, moving somewhere else. A piece of music might “pass through” one or more keys before reaching a “modulation”. 


At whatever point in the score you are trying to work out what the key is, remember that

  • a key is most often established with a dominant-tonic relationship (look for chords V-I)
  • Chord V7 (V with an added 7th above the root) is often used in place of V


This means that you need to:

  • look carefully at the supporting harmony (don’t rely on the melody alone!)
  • make a note of which chords are used (e.g. D major, G minor…)
  • find the key which has those chords as I and V (the chords should be next to each other).
  • if more than one key uses those chords, look at the other melody notes for more clues.


Let’s look at a real life example. This is from a flute sonata by Donizetti. Let's try to answer these questions.

  • What key does the piece start in?
  • What key has it modulated to by bar 16?






  • The piece starts in C minor.
    • The first chord is C minor, which is i in C minor (a primary chord), but vi in Eb major (not a primary chord), which rules out Eb major.
    • Bar 3 contains nothing but Gs. Since G is the dominant in C minor, this bar would be chord V in C minor.
    • Up until bar 11, we see a lot of B naturals, as would be expected in C minor (the 7th degree of the scale is raised).
  • The piece modulates to Eb major by bar 16.
    • The modulation starts in bar 14 with the introduction of the B flat instead of B natural. The notes of the piano chords are Bb-D-F-Ab, which is chord V7 in Eb major. (It would be chord VII7 in C minor – the major chord formed in that key by not sharpening the 7th degree of the scale).
    • In bar 16, the Eb major chord is stated as the new tonic – the music has modulated to the relative major key of Eb major. (See the next unit for more on “7” chords).
    • In bars 17 and 18, the E natural and F sharp notes are chromatic passing notes. They don’t affect the key of the piece as they are just a form of melodic decoration. (See the unit on melodic decoration in part A for more on this).
  • To sum up, this piece is in C minor then modulates to the relative major key of Eb.


Tips for Working out Modulations

Although it's possible to modulate to any other key from any starting key, in practice, there are only a small number of likely modulations, especially in Baroque and Classical music. These are:

  • Tonic to dominant
  • Tonic to subdominant
  • To relative minor/major

It is usually worth assuming that the modulation is one of these, and then testing out which one it is by looking at the underlying harmony.

You should also scan the score for accidentals, then work out which keys those accidentals belong to. In the Donizetti piece above, the B naturals at the start were a clue that the piece starts in C minor. This means you should keep on looking at B's to see when the naturals stop being used. Another example could be a piece which starts in C major, but then F#s are introduced - this should lead you to the fact that the likely modulation is to the dominant, G major.

Don't rely on accidentals blindly though. As in the above piece, they are often used simply for decoration. Accidentals which are used for chromatic melodic decoration will not affect the key. 


Awkward Key Questions

When working out the key or modulations of a piece of music, don’t forget that sometimes you will have to transpose parts (or read awkward clefs) to work out what notes are being played.  It can be confusing to work out the key of an orchestral piece, so let’s have a go!

 This is the beginning of the slow movement of Brahms’ 2nd symphony.




  • First look at the key signatures here – it’s easier to check the key signature of the non-transposing instruments to get started. Here, the non-transposing instruments (e.g. violin, flute, oboe, cello…) have five sharps – so is it B major or G# minor?

  • Next look at the notes played by the low-pitched instruments. Usually, the lower pitched instruments will provide a bass line which will help us determine the chords.
    The double bass and bass tuba both have long F#s – sustained (or repeated) notes are always a good indication of key. The cello and bassoon, on the other hand, have a fragment of a melody. Melodies are less useful for determining the key.
    In this case, the double bass and tuba look like good candidates for showing us what the key is, so we will focus on them.
    The next notes played by the double bass and bass tuba are D# and B. Clearly, the piece is in B major, not G# minor. Why? Because F# is the root of the dominant (V) of B major, and D#/B are part of the tonic triad (I). (In G# minor, F# is the root of the unsharpened 7th degree of the scale (VII), which is never a likely chord for the opening of a piece.) Remember: always look for the V-I relationship.
  • Always look for the parts which move slowly or by leaps (i.e. intervals larger than a 2nd) – these are likely to be the parts which are providing the harmony.
    Parts which move mostly quickly or by step, are more likely to be supplying the melody. We need to analyse the harmony of a piece to determine its key.



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