For those who are not sure what a Scale Overlay is or what the term means then let's start there. A "scale overlay" is a guitar diagrammatic image of a scale on the fingerboard that includes the notes of two scales simultaneously. 

When we do an "overlay" we are not doing any note replacement with the scale, but rather we are including all of the tones from two different scales to be able to see the core differences.

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Some teachers might have you believe that seeing any kind of progress after viewing a scale overlay cannot occur. And, if we were pursuing that one result, they'd pretty much be correct.

However, what is more important about the overlay process is how a scale overlay will offer a musician something of a shortcut to cherry-pick the exact notes that end up producing a change to the overall sound.

For our example, the Major sound will be highlighted and the we will isolate a new series of notes that produce the Minor sound.Our goal will be to know what notes specifically operate to produce the difference in color from major to minor.

The best part about this exercise is that all that you’ll need to do is focus on this information once. The fact of the matter is, if you’ve ever tried to make sense of major and minor scales but you ran into problems, then you need to know this information. 

If you cannot make sense of scales and you’re an older guitar player, then you need to learn about this concept known as the Major and Minor Parallel scales. 

THE FIRST WAY WE LEARN SCALES:
When we are younger (and we don't yet know anything much about scales), we will typically learn about them by way of how the scale "functions" within a song as a shape. 

We won't necessarily learn how the scale functions in regard to music theory, or how the scale is related to the key signature of the underlying music, but more so how the scale actually sits upon the neck geometrically, (as a shape). Study the scale shapes shown below;

fig. (1). "C Major" scale:

fig. (2). "C Minor" scale:

fig. (3). "C Major and Minor" Pattern Overlay:

NOTE LEGEND: 

• Red dots = Root Notes (examples show "C")
  
• Blue dots =  2nd, 4th and 5th (static tones - no change)
  
• Orange dots = Core Major tones (create Major)
  
• Purple dots = Core Minor tones (create Minor)
  

SONG LEARNING:
Songs are critical for learning most of our early principles on the guitar, (technique, rhythm, note locations, etc.). When it comes to learning scales we will tend to take a small piece of a scale that we learned from a song and use it in very generic ways. The application of those tones is often very limited.

Normally, we won’t fully comprehend how that “piece” of a scale we learned from a song truly  works with regard to theory, (or even more so, why that piece of the scale worked where it was played). Rather, we tend to only focus on how the scale sits on the guitar neck and how it sounds when it is used in a simple manner.

TONALITY:
To further you playing skill and your musical knowledge, we will need to study how you can understand the most common scale concept on Earth, the "Major and Minor" tonality. 

Tonality is actually quite simple. It is the "color" of whether a scale section being employed is of the effect of Major or Minor in a musical phrase. 

If "Major" is in use, we have the tones shown above in "fig. 1." But, if "Minor" is applied, we will see a shift occur. That shift will be involving the use of a new group of notes that promote the minor effect. In our example above (see fig. 2), these notes will be, "Eb, Ab and Bb."

MAJOR and MINOR:
Using the parallel major and minor idea is something that takes time (and skill). For a guitarist to be able to make sense of using both major and minor musical sound it all starts with being able to properly hear the effect of these scales and how they are functioning in a musical sense. 

Let's begin by using a very simple practice technique that will allow you to bring in the color of each major and minor sound. Plus, we can use this technique to be able to start creating music with these different sounds.

PARALLEL MAJOR /MINOR (2-BAR):

Example 1). The first example that I want to go over is based upon applying the sound of both the major scale notes and the minor scale notes across 4 measures, but the shift will cover 2-bars, (allowing for plenty of time to change notes). 

In the example below, the major scale will work for measures one and two (on the “C” Major Chord), and the Minor notes will need to be added on the 3rd and 4th measures to cover the “Ab” Major chord. Play through the example below.

The final exercise is going to go over switching between scale tones faster. This time, we’ll switch between Major and Minor scale notes on every measure across a four bar progression. 

Doing this is a little more challenging, but if you take your time and learn the notes that have to change, you’ll find that the sound will become more manageable and it will also start to become easier to do.

Example 2). The final exercise applies the sound of both the major scale notes and minor scale notes but it will switch them on every measure. 

In the example below, the major scale will work for measure one, (on the “C” Major Chord), and the Minor notes will need to be applied on measure two, (for the “Eb Major” chord). 

The Major notes will have to come back on measure three (for the “F Major” chord). And finally, you’ll need to apply the Minor scale notes on that last measure (to cover the “Ab Major”).