A concept we can use for relating things, is that for every chord we learn, we link a scale or scales to it. This way we can see how a scale is wrapped around a chord or fills a chord in, & how a chord is inside a scale or is a hollowed-out scale.
Take a look at both the chord & the scale. We can see that all of the tones of the chord are inside the scale, while the scale tones not in the chord, wrap around the chord. In this way, a scale is like a filled-in chord, while a chord is an extracted piece ofa scale.
When we use a flat like this, it means we are comparing or parelleling to what is normal to the root. What is normal means what is derived from the Major scale for the root.
For E minor, the G is a flatted 3, because in the key of E Major (the E Major scale), the 3 is G#. Likewise, since the 7 in E Major is D#, D is the flatted 7 (b7) to E.
For a further discussion of this, take a look at Derivative vs. Parallel.
Notice that this is the same tone group as E minor, yet we are naming it G Major. We are linking it in the same way we linked Em to Em - we link the G6 chord to the G Major Pentatonic.
Another point here is that the G Major Pentatonic is the 'G Major scale without the 4th & the 7th tone.' Note, that even though we can parallel (compare) the pentatonic to the major, the pentatonic is its own scale, in its own right.
The pentatonic scale system has 5 tones. In this lesson, we started this group of tones (E, G, A, B, D) on/from two tones, the E & G. When we did this, we are playing the E minor pentatonic and the G Major pentatonic guitar scales.
We can start this grouping on any of the other 3 as well. When we do this, we are playing the following guitar scales:
From the A, the scale is called the Vietnamese Scale (R 2 4 5 b7)
From the B, the scale is called the Malkos Raga (R b3 4 b6 b7)
From the D, the scale is called the Bac Scale (R 2 4 5 6)
Now, in the West, this isn't common. Yet, we see the Eastern names, and now we know this: any scale can be started on any tone of the scale, and we have a new scale. These are called modes (although the term modes most often applies to 7 tone scales, we can use the same concept for any scale or tone group). Modes are another way of saying guitar scales (division of an octave).
The rule with scales is that the number of tones in any scale is the minimum number of names that it can have.