Identifying the key at the beginning of a piece is straightforward - you can look at the key signature, as well as the first and last chord of the piece.
However, identifying the key in the middle of a piece is not straightforward and can be difficult.
Here are four steps to help you identify important key areas in the middle of a piece.
1) Are there any recurring accidentals? What key do the accidental suggest?
If you see certain accidentals recurring in a given section of a piece, they could indicate a key change. Identify these accidentals and examine the key.
Below is an excerpt taken from Schubert's Impromptu No. 2 in E-flat major, D. 899.
The key signature contains F# and C# - which suggests D major or B minor as the tonic key.
These six measures contain the recurring accidentals: E# and G#.
What is a key that contains F#, C#, E# and G#?
F#, C# and G# is the key signature of A major or F# minor. The E# could be explained as the leading tone (the raised 7th scale degree) in F# minor.
Hence, this passage is in F# minor.
2) Are there any perfect authentic cadences?
A cadence is a harmonic formula that ends a melodic phrase. There are many types of cadences, but the strongest cadence that could end a phrase is the perfect authentic cadence (PAC).
Perfect authentic cadence contains a root position V or V7 chord moving to a root position I chord, with scale degree 1 in the top voice of the I chord. The V or V7 chord is typically embellished with a cadential 6/4.
If you find a PAC in a key other than the tonic key, then that PAC confirms that a modulation has taken place.
The example below is taken from Mozart's Piano Sonata No. 5, K 283: III.
The tonic of the piece is G major.
Upon listening to the excerpt or completing a Roman numeral analysis, mm. 137 - 138 contains a PAC in E minor.
B D# F# A is V7 in E major or E minor.
E G B is I in E minor.
Hence, the piece has modulate to E minor, from the tonic key of G major.
3) Does the harmonic progression orient towards a certain tonic?
Sometimes, modulation can occur in the middle of the piece without an obvious PAC in the new key. In such a case, examine the tonal syntax (that means, the ordering of the chords) to identify the key.
Below is an excerpt taken from Mozart's piano Sonata No. 8, K. 310, II.
The tonic of the piece is F major.
German augmented 6th harmony often occurs before cadential V chords. Hence, the next chord on m. 43, downbeat, is likely an important V chord in some new key that the piece has modulated to.
This chord - the chord after german augmented 6th, on the downbeat of m. 43 - is A C# E, which is V in D minor.
Indeed, if we look zoom out an examine the whole progression, mm. 42 - 43 contains a cadence in the key of D minor.
Therefore, the piece has modulated to D minor, from the tonic key of F major.
4) Can you hear or sing the key tonic of the excerpt?
Lastly, if you still cannot identify the new key after using all the advice above, listen or sing the passage. Hear if a certain note in the new key sounds the most stable - that note is the tonic of the new key.
Try to sing or play this melody below from Handel's Giulio Cesare. The red numbers indicate the scale degree of the pitch in G# minor.
The tonic of this aria is E major.
When you sing this melody, you will hear that G# (that is, pitches with red "1" above them) sounds the most stable. Therefore the tonic of this melody is G# and this section is in G# minor.
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