In algebra, X is an unknown
A spurt is a drip under pressure
So an expert is an unknown drip under pressure
A joke told by an Engineer

Once a person is playing the MAJOR CHORD PROGRESSION and the BOX SLANT BOX without thinking about it, they will start thinking about the notes NOT in the MAJOR SCALE. Most styles of music don't bother moving past what you should already know.
If you are to the point that you are playing the MAJOR SCALE over the entire neck with ease, they you will start to recognize it and quickly tire of it.
Few reach this point.
If you are here, it is time to start filling in some of the blanks that we skipped. If you have the previous lessons under your belt, you already know WHERE the things are that we will be talking about and half the battle is won already.
Let us examine the six notes of BOX ONE
Let us fill in the empty area between these numbered notes
Let us examine the notes of SLANT
And again with the empty notes filled in
Let us examine the notes of BOX TWO
And BOX TWO with the empty notes filled in
These are the rest of the notes in music. This is where many people give up, because it all seems so confusing. It is really quite simple, it just happens that a lot of people seem to feel better about themselves if you are confused.
If you look at the chart below, you will see the names of the notes you have been playing and the ones you have not. If you have been playing for a while, you may have heard of some of these for years and not known what they were. Strip all the names away, and it is just an interval from the ROOT or KEY.
 0 ROOT  P1 / d2 Diminished Second / Perfect Unison / Tonic / Key
 1 Minor Second  m2  / A1 Augmented Unison / (9th an octave above)
 2 Major Second   M2 / d3 Diminished Third 
 3 Minor Third   m3 /A2 Augmented Second / (11th an octave above) 
 4 Major Third  M3 / d4 Diminished Fourth
 5 Perfect Fourth  P4 / A3  Augmented Fourth 
 6 Tri-Tone TT / d5 / A4  Diminished Fifth / Augmented Fourth /
(13th an octave above) 
 7 Perfect Fifth  P5 / d6  Diminished Sixth
 8 Minor Sixth   m6 / A5  Augmented Fifth
 9 Major Sixth  M6 / d7  Diminished Seventh
 10 Minor Seventh  m7 / A6  Augmented Sixth
 11 Major Seventh  M7 / d8  Diminished Octave
 12 Perfect Octave  P8 / A7  Augmented Seventh
Do not get confused when people start throwing around terms that you do not know. They are just using different language to describe something that you already know.
You will also want to learn your intervals.
Here is a great tool to start with