If play a chord progression on my guitar and all notes are neither no sharp nor flat, but the C chord itself is not in the progression, is the song considered in the the key of C?
 
If so, and I play the lead with all notes in the key C, it would be the Ionian mode? 
 
Thanks

could be in Am...

No sharps or flats does not instantly mean it's in the "relative modes" of C. There could possibly be notes being ommited that are flat/sharp.
 
However, yeah... that's a good indication it's in the key of C Major or one of it's relative modes.
 
Those would be...
 
C Ionian (Major scale)
D Dorian
E Phrygian
F Lydian
G Mixolydian
A Aeolian (natural minor)
B Locrian
 
 
As far as it possibly being "A minor" as noted above, without sharps or flats it would have to be exclusively A natural minor (Aeolian mode). Melodic and Harmonic minors cause sharps/flats in C Major.
 
What is your root chord/chord progression?

And to make it a little simpler...
 
Play the notes of C major over your chord progression. Do any of them sound "sour"? If not it's likely in the key of C (or one of it's relative modes).
 
As you solo revolve the solo around the root note and the 5th note of the chord using the notes of C major. Those will be your sweetest notes (with the root being the resolve).
 
The "less" sweet but still sweet notes will be your pentatonic scales (major/minor depending on the quality of the chord... so major use major pent and minor chord use minor pentatonic).
 
The remaining notes will be ones you just travel through to get to those other notes or bend up from to those other notes.
 
The notes in between all THOSE notes can be used in a "chromatic" way so you can blow through them quick or hang on them for tension/dissonance on yoru way to the sweet notes (like the blues scales).
 
Okay... the first paragraph was simple. The rest may take some doin'... but that's a quick guide to improv in a specific key. It does not cover chord by chord type progressions that twist keys up or account for melodic/harmonic minor wackiness.

And another easy foolproof tip is if you are using wackier chords like in your previous thread as you solo you can safely land on/bend up to any notes in the current chord and it'll usually sound good.
 
Getting the hang of all this and how you execute it all becomes the big challenge at that point but you'll not have to worry about "bad" notes anymore.

I was using C as an example.
 
 
But in that example, the root of the starting chord is an F, let's say. There could also be an Am or Am7 in the progression. 
 
This is where I get a little confused. All the notes of the chords could all be in the key of C major. But the starting chord's root is an F. I would not play in the key of F - Ionian because there is a B note in one or more of the chords in the progression. However, F - Lydian would work. So would I say I am in the key of F? Then the Lydian mode is what I am playing?
 
But saying I am in the key of F does not make sense if there isn't a Bb, right?
 
C - Ionian, F - Lydian, A- Aeolian have all the same notes in the scale. So does it matter? I think so. How else do we communicate? lol
 
So am I playing in C or F?
 
Thanks for the help!
 
 --Edit--
Since I mentioned Flying in a Blue Dream in the other thread... The opening chord has C as its root but the chord itself has an F#. That is why, in this case, C - Lydian sounds good with it. But to say he is in the key of C is not correct. Otherwise, the scale he plays (predominantly) could not be called Lydian. 
 
So I think in my example, it would be key of F and the F - Lydian is what sounds best.
 
Now that I type this out loud (or whatever), it doesn't sounds right to me lol How can I be in the key of F?
 
I don't understand keys and modes (as far as naming goes). lol
Thanks
 

Nope. F is the 4th step in C Major/Ionian. So as you solo over the F you use F Lydian for your solo because Lydian is the mode that appears on the 4th step of a Major/Ionian scale.
 
You have not changed keys, none of notes have changed... the only thing that has happened is you are moving the root note you are using. That's all modes are. Taking the Major scale and starting on a different note.
 
I = Ionian (Natural Major Scale/Major Chord)
II = Dorian (minor chord)
III = Phrygian (minor chord)
IV = Lydian (Major chord)
V = Mixolydian (Major chord)
VI = Aeolian (Natural minor/Minor chord)
VII = Locrian (diminished chord)
 
As for 7th chords this is how they appear in the Major scale (because of where the 7th note appears in each of the modes).
 
I = Ionian (Natural Major Scale/Major Chord) Major 7th
II = Dorian (minor chord)                            Minor 7th
III = Phrygian (minor chord)                         Minor 7th
IV = Lydian (Major chord)                            Major 7th
V = Mixolydian (Major chord)                        Dominant 7th
VI = Aeolian (Natural minor/Minor chord)       Minor 7th
VII = Locrian (diminished chord)                   Diminished 7th
 
Hopefully that helps a bit.

BTW that is the most basic/literal interpretation of the modal system. It can be bent and twisted using various means but that's the main gridwork/foundation of the natural keys.
 

If your new to theory, this is a great tool and the users guide is full of great info
 
http://randscullard.com/CircleOfFifths/

BobF
If your new to theory, this is a great tool and the users guide is full of great info
 
http://randscullard.com/CircleOfFifths/

But what if you're backwards and prefer the Circle Of Fourths???