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Now consider either the C or Am chords in bars 14 and 15. Here the choices of potential sevenths, following conventional harmony, are more circumscribed. C7 might be questionable, because it doesn't resolve like a dominant (you could call the move the Am a "deceptive" progression, but that might be iffy.) With the Am if you used a major seventh, it would be a G#, which is contrary to the G natural of a key of E minor, and so might not work well.

The point is to be able to analyze the big picture of the harmony in a song, understand chord functions, and they use added and extension tones in a way that works with the harmony. The C and Am chords are not functioning as dominants (nor tonics) and are not part of root progression by descending fifths, so selecting tones that would be appropriate for tonic or dominant chords might not work.

Now consider either the C or Am chords in bars 14 and 15. Here the choices of potential sevenths, following conventional harmony, are more circumscribed. C7 might be questionable, because it doesn't resolve like a dominant (you could call the move the Am a "deceptive" progression, but that might be iffy.) With the Am if you used a major seventh, it would be a G#, which is contrary to the G natural of a key of E minor, and so might not work well. It could work as some type of chromatic non-chord tone.

The point is to be able to analyze the big picture of the harmony in a song, understand chord functions, and then use added and extension tones in a way that works with that harmony. You should have an idea of what the choices mean harmonically.

I'm glad you posted a link to a lead sheet...

...it's so much easier to discuss actual music.

In that four bar passage, there is the plain G major triad given.

What is my 7 if a chord does not specify the 7th?

So, essentially you want to know if you should play a minor seventh or a major seventh over that plain G major triad.

In this particular case you could easier play either type of seven. IMO this is a good example of how the chord/scale system fails. Instead of matching a chord to a scale, you really want to understand harmonic function, and why in this case either seventh type will work.

If you play an F#, you will have a major seventh. That will match the key signature, making it a fairly obvious choice. In terms of function you could then say the D7 G C passage is either Em: V7/III III VI or temporarily shift to G as G: V7 I IV. Either functional analysis is fine. The song clearly works around the shifting minor and relative major keys, and you would say the G chord gets tonicized.

But, you could also play an F natural over the G major triad, that would be a minor seventh. In terms of function that would then be Em:V7/iii V7/VI VI. There would be a sequence of two dominant seventh chords, and you would say the C major triad gets tonicized.

After that particular passage the line continues to end on B7 which definitively brings us back to E minor for the key.

The big picture is an "A" section that is in E minor. Whether you use a major or minor seventh on the G major triad, and which chord get tonicized as a result, doesn't matter to the bigger picture of playing something in E minor. The choice of either would be up to you.

Now consider either the C or Am chords in bars 14 and 15. Here the choices of potential sevenths, following conventional harmony, are more circumscribed. C7 might be questionable, because it doesn't resolve like a dominant (you could call the move the Am a "deceptive" progression, but that might be iffy.) With the Am if you used a major seventh, it would be a G#, which is contrary to the G natural of a key of E minor, and so might not work well.

The point is to be able to analyze the big picture of the harmony in a song, understand chord functions, and they use added and extension tones in a way that works with the harmony. The C and Am chords are not functioning as dominants (nor tonics) and are not part of root progression by descending fifths, so selecting tones that would be appropriate for tonic or dominant chords might not work.

It depends on what the next chord is.

"Guide tones" is just another way to say "voice leading" and you need to move to another chord to really say what happens moving tone to tone.

The chord third and seventh are the usual tones when talking about "guide tones", so...

• you have the third to handle as any other third as "guide tone"
• the ostensible seventh of the chord can be treated as any other non-chord tone, for example as a passing tone, neighbor tone, part of an enclosure to a following chord tone, etc.
• you can decide to modified the harmony on-the-fly, for example is the following chord were Cm you could add a minor seventh to a G major triad to make G7, where, in terms of "guide tones", a seventh resolves to a third
• you could use another chord tone as your "guide", for example in a progression of G Cm, you can use tone G as the guide, the root of G then becomes the fifth of Cm.

If I decide to just throw in a minor or major 7th, am I changing the function of the chord?

Depends how you mean "function." If you mean the standard idea of functional harmony, where functions are pre-dominant, dominant, tonic, then "no" adding a seventh won't change the function. In other words d(dim) G Cm and dmin7b5 G7 Cm6 are both functionally pre-dominant to dominant to tonic regardless of extensions or added tones.

But, as with anything, the devil is in the details. With a progression like d(dim) G Cm you could add a minor seventh to the G chord (add an F natural) and handle it as a chord tone with standard resolution to the third of Cm. But, you could also use an F# over the G chord and treat it like a non-chord tone neighbor tone to the chord root G. The first scenario is the obvious way to set up/confirm a dominant to tonic function, but the second scenario using F# certainly doesn't preclude dominant to tonic function.

It depends on what the next chord is.

"Guide tones" is just another way to say "voice leading" and you need to move to another chord to really say what happens moving tone to tone.

The chord third and seventh are the usual tones when talking about "guide tones", so...

• you have the third to handle as any other third as "guide tone"
• the ostensible seventh of the chord can be treated as any other non-chord tone, for example as a passing tone, neighbor tone, part of an enclosure to a following chord tone, etc.
• you can decide to modified the harmony on-the-fly, for example is the following chord were Cm you could add a minor seventh to a G major triad to make G7, where, in terms of "guide tones", a seventh resolves to a third
• you could use another chord tone as your "guide", for example in a progression of G Cm, you can use tone G as the guide, the root of G then becomes the fifth of Cm.

If I decide to just throw in a minor or major 7th, am I changing the function of the chord?

Depends how you mean "function." If you mean the standard idea of functional harmony, where functions are pre-dominant, dominant, tonic, then "no" adding a seventh won't change the function. In other words d(dim) G Cm and dmin7b5 G7 Cm6 are both functionally pre-dominant to dominant to tonic regardless of extensions or added tones.

But, as with anything, the devil is in the details. With a progression like d(dim) G Cm you could add a minor seventh to the G chord (add an F natural) and handle it as a chord tone with standard resolution to the third of Cm. But, you could also use an F# over the G chord and treat it like a non-chord tone neighbor tone to the chord root G. The first scenario is the obvious way to set up/confirm a dominant to tonic function, but the second scenario using F# certainly doesn't preclude dominant to tonic function.

I think the fourth bar of a standard 12 bar blues provides a good example to consider. For a blues in G the fourth bar will be a G chord. Some lead sheets might give it as plain G, others G7, and many people might read G but play G7, the next bar will be a C chord. You could describe that change as a tonicization of C when preceded by G7, some might even call it modulation (even though that doesn't make sense.) Functionally it's just a move from tonic to subdominant. When it moves back to the tonic you can even call that a tonic prolongation. But, the important point is that the overall harmonic, functional structure of that 12 bar blues isn't going to change just because you added a minor seventh to the tonic chord. The larger harmonic structure is what will determine function, not the minutiae of embellishing tones.

In other words, you can't really say anything definitely about chord function if you give only one base triad an then consider one added or extending tone.