@Stuartw Take a basic song with a straightforward progression. That song is in a specific key - let’s use the key of G major for an example. In which case everything will derive from and can be referenced back to the G major scale:
G, A, B, C, D, E, F#
The chords within the song will be chords found within the key.
G, Am, Bm, C, D, Em, F#dim
The notes played by any instrument or any singer will be chosen from some or all of the notes from the G major scale, perhaps spread across more than one octave. The bass may be playing notes way down low. The singer may be playing notes up high. The guitar or keyboard may be playing notes (in the form of chords or perhaps triads or arpeggiated chords) somewhere between.
When it comes time for an instrumental break, if a guitar is going to take a lead solo, the notes of that solo will come from the G major scale also. What if the soloist wants to play something more melodic and musical than just randomly happened upon notes from the scale? Knowledge of chord construction and chord tones is where that musician can find some answers.
Let’s say the progression for the instrumental break is:
| G | D | Em | C | Am | Bm C | D | G |
Break down those chords to their constituent notes (triad form).
G = G, B, D
D = D, F#, A
Em = E, G, B
C = C, E, G
Am = A, C, E
Bm = B, D, F
Each and every note for each and every chord comes from within the G major scale. The progression is in the key of G so that should be no surprise. What options does the soloist have for each chord?
When the G chord is playing the soloist can play G, B and D and it will sound so, so good. Those three notes are chord tones. They are the notes of the triad. Perhaps those notes could be located, identified and played by moving to one of the triad shapes!
When the D chord is playing the soloist can play D, F# and A and it will sound so, so good. Those three notes are chord tones. They are the notes of the triad. Perhaps those notes could be located, identified and played by moving to one of the triad shapes!
When the Em chord is playing the soloist can play E, G and B and it will sound so, so good. Those three notes are chord tones. They are the notes of the triad. Perhaps those notes could be located, identified and played by moving to one of the triad shapes!
I will not copy / paste identical looking paragraphs for the other chords. That should explain sufficiently.
What if the soloists only plays one or more of those chord tones as the chords come along and never strays from those notes for each chord? Well, it may just work and be the most inspired solo ever played. Or, it may sound a little predictable with little tension and release, little in the way of movement and melodic surprise. So the soloist may think of the chord tones as a safe and secure set of notes but know that creative expression and interesting improvisation lies beyond those parameters and involves stretching out to use the chord tones mixed in with other notes from the G major scale.
The triad shapes can give that platform and a sure-fire means of knowing where the chord tones. Visualising the triads within scale patterns allows pushing beyond the obvious and offers a fuller, richer and more satisfying means of playing a lead part over a chord progression.
And, to directly answer your question … if you do not know the chord progression, you do not know which notes of the scale are the chord tones and you cannot improvise a solo as described above. Your solo may sound okay if you play from the G major scale but your chances of it all sounding okay all of the time without hitting any ‘sour’ notes or without it sounding somewhat random and high.
