Hi, I have a question that is probably pretty dumb for you educated musicians. If I or my guitarist plays a chord, must all the notes be in key or does only the root matter when it comes to staying in key?
Thanks!
Well, this may be a little confusing, but I think it's important to understand that sometimes even the root note can be from outside the key.
Let's use the key of C for example, because it's all "white keys" and the minute we introduce a sharp or flat it's clear we're "outside" the scale.
Let's say you were going to play a progression moving from G7 to C. One substitution we could make would be to use a Db7 in place of that G7. This would not be common in rock, but it's almost a cliché in jazz. It works for various reasons, but one of the most easily-grasped is that G7 and Db7 have two notes in common (B and F).
G7= G, B, D, F
Db7= Db, F, Ab, B
All right, now forget everything I just said. Don't try to digest it yet. You're not ready for that. To get to that, first you need to have a solid handle on "diatonic" harmony. Diatonic means that all of the notes involved are in the key of the song, which relates to your question.
In fact, the short answer to your question is: no, all the notes do not have to be in the key, but if they are, the progression is termed "diatonic".
If you take every note of the major scale and assign it a roman numeral, and keep to diatonic chords, you get this pattern:
I major
ii minor
iii minor
IV major
V major
vi minor
vii diminished
Notice that major chords got a capital roman numeral, where minors and diminished are in lower case. This is a common shorthand, so you can just say I, ii, iii, IV, etc.
That list is definitely worth committing to memory. Once you have that down, you can start learning the theory behind the juicier notes.