Question about the key of a chord progression

You can’t always tell by just looking at the chords – context can make a difference, so without hearing it, I couldn’t be certain. I’ll take a stab at it though with probability. Chord progressions like this which use all major chords are usually used in pop or rock songs and are rarely complex. My guess is the key of C major with the D just being an outside chord. Let’s break down my reasoning.

Firstly, there is no common key (none I could even think of) that contain four major chords. Remember, this is unlikely to be something complex. This leaves us with the possibility that this progression is either switching keys or is mostly in one key with an added outside chord (quite common).

Let’s for a moment assume it’s switching between two keys. Quite plausible with these chords but not so much with the order you have written them. Take G and D for example. If this was a two chord song using just those two chords then you’d likely just think of it as a I-V in G major or possibly a I-IV in D major. The same could be said for F and C. This could be a I-V in F or I-IV in C. Now lets’s look at a chord sequence that goes something like this, played one bar per chord.

F/// C/// F/// C/// F/// C///

G/// D/// G/// D/// G/// D///

You can pretty much hear the key change – six bars of F major and then six bars of G major.

Now let’s play the chords something like this.

F/// //// C/G/ D/// F/// /// C/G/ D///

Now you don’t quite hear the key change so much, it’s a bit less clear and obvious – even a bit messy sounding. Is it even changing key? probably not, it’s just going off piste a bit.

Now play the same chords but change the order and the duration.

C/// F/ G/ C/// F/ G/ C/// G/// F/// D/// C/// F/ G/ C

This time everything sounds pretty much like C major but with a D thrown in for a bit of decoration.

So it all depends on context. With the chord order you have, assuming it is played one bar per chord, we can’t hear any obvious key change going on. This leaves us with two possible keys – C major (C,F and G) or G major (G,C and D).

More often than not, the chord that feels most stable to end the song with is most likely to feel like the home key. If I play your sequence a few times and end on the G it just feels slightly less stable than ending on the C. Obviously I’m playing all chords with equal duration so this could change things if played differently, but overall, that’s pretty much why I have come up with C major.

Does that help?