I wrote a song, with a simple four-chord progression. The chords are A - G - D - E and back to the A. Now, I am trying to identify the key of the song, and I have run into a problem. I originally thought the key of A, however the G natural in the G chord is not in the key of A. I then thought the key of D, however the G♯ in the E chord is not in the key of D. using an online key finder bore the results of both keys A and D. So which is more accurate in terms of musical theory? Is there one that is more correct? Why are there exceptions outside of a key?
Great question. I'll give an answer after a little bit of 'secret' music theory. Then I'll give a better answer after explaining how perception is so important.
All key signatures for diatonic scales have three major chords, and they are each separated by a perfect fourth.
For example, in the key of C Major, there are three major chords G, C, and F. Note that when you arrange these chords in "circle of fourths order", C is in the middle, and C is also the root. This is a property of all diatonic scales--there are three major chords and in circle of fourths order the middle chord is the root.
Since you're playing the chords A G D E, these can be rearranged to E A D G in circle of fourths order.
I like to think of chords E A D G as a 'fusion' or 'superposition' of two keys, A and D : you're playing in the key of A with the chords E A D but you're also playing in the key of D with chords A D G. It's a fusion of keys, also a fusion of two sets of chords.
Now the perceptual effect of what you are playing is very cool. When you play the first three chords A G D, it sounds like it is in the key of D because of the property I stated above above. But then when you play E, one perceives a key change and tries to make sense of what happened. Remembering two chords back, recalling the chord G, one subconsciously recognizes that G and E do not appear together in a single diatonic scale, so we must be in a new scale. Remembering one chord back, we hear E, and realize that D and E do indeed go together in a key. So we are definitely in a new key, we appear to be on stable ground, and these chords exist precisely in the key of A. So the mind is less confused at this point, fairly certain that we are in the key of A. Now, the chord A is coming up. A is definitely in the key of A, so when we hear the chord A, the brain feels pleased with itself for predicting the future. But then immediately after the mind feels comfortable, G appears, which is in a different key! The brain is confused again. And the cycle continues.
This 'perceptual rollercoaster' kind of effect is exactly why I think the chord progression is so satisfying. The mind is constantly alternating between a state of confusion and state of understanding as the key of the song is alternating between the key of D and the key of A.
Put it in key A. That's I, and D is IV, while E is V. The slightly awkward G is said to be a borrowed chord, from, in this case, A minor, the parallel key.
It's theory, an observation, not a rule, and obviously it works, not only in your song, but many, many others.
The reason A seems better is that the E at the end of four bars (I guess) is the dominant, taking the song back to its first chord. If the key was D, it's in the middle, with no V before it, and seems like the middle of a journey rather than arriving safely back home.
Safest way to determine a song's key is to play through some of it, and stop on each chord. The one that feels most final is usually the song's key.
Sounds like the key-centre is A major. The bVII chord is very commonly used. You can just accept this as a fact, or you can justify it through 'borrowing'. Best, I think, to get your head round the idea that the diatonic (in-scale) notes of a key are a framework, not a restriction. Chromatic chords ARE 'allowed'. Someone is teaching beginners otherwise, and they should stop!
From what you say, it sounds to me that your song is in A major. The G natural just adds a bit of flavour and variety which can be attractive. Using accidentals can make it sound interesting as long as you don't wander too far from your tonal base and end up modulating somewhere you didn't intend.
At first glance this seems very much in A major - with G as a borrowed chord - because of the full cadence (
D E (A)). However, it is very important to look at the rhythmic and structural components of the song to decide the correct key.
Whether the chord progression starts on a heavy or a light beat - and which chords land on the heavier beats - can change a lot about which key the ear perceives.
Which beats are perceived as weak or strong is decided by the form of the melody, the rhythmic section, the bass and even convention (how the ear identifies patterns in music). The best way to know is to just feel it. In western music the heaviest beats - in any 4 bar phrase - are usually the first beat of the 1st and 3rd bar.
G start on light beats (upbeats),
D on the heaviest beat and then
E on a weak beat this chord progression will be perceived to be in D major, making E the borrowed chord.
Where the chord progression appears in the song is also important. If this progression would appear as a chorus after a verse in D major it could be perceived as D major.
In rock music it is common to flat the 7th to give you a major triad (i.e. VII) instead of diminished (viiº) so you could call it the key of A; BUT there are things which are referred to as adopted chords and this would be replacing any given chord with its parallel minor/major. although I'm not sure if you'd call it that. anyway, you can call it the key of D major with an adopted II. But I would keep it with the key of A considering the E would act as the 5th and resolve to the 1 and get that nice cadence.
A theory to justify the G is that the main chord progression is A, D, E, A (that is, I, IV, V, I). The G chord is the IV chord from the key of D, so the G is the IV of the IV, sometimes notated as IV/IV. This progression I, IV/IV, IV, V, I is fairly tame as far as chromatics go.
To understand the IV to I motion a little better, the borrowed chord can also be thought of as a double suspension of the D chord, with the note F# raised to G and the note A raised to B. From this point of view, it's not really a separate chord, but the D-chord-to-be.