Without question, the most commonly used scale in western guitar music is the minor Pentatonic scale. This scale is mostly used in blues, jazz and rock music. It is called the Pentatonic because it is a scale with only 5 notes (The Greek work 'Pente' meaning 5).  
The formula for the minor Pentatonic scale is 1,3b,4,5,7b, used against the Major Scale.

Let's work with the A major scale since that is the one we work with in these lessons the most. From the Major Scale Lesson, we know the A Major scale contains these notes: A,B,C#,D,E,F#,G#

Now, lets find the notes in the A Minor Pentatonic scale using the formula: 1,3b,4,5,7b

Take the first note of the scale (A)
Then take the 3rd note and flat it (C)
Take the 4th note (D)
Take the 5th note (E)
Take the 7th note and flat it (G)
-----------------------------------------
The notes we end up with are: A C D E G

Here it is plotted on the fretboard diagram:

E ||-F-|----|-G-|----|-A-|----|-B-|-C-|----|-D-|----|-E-|-F-|----|-G-| 
B ||-C-|----|-D-|----|-E-|-F-|----|-G-|----|-A-|----|-B-|-C-|----|-D-| 
G ||----|-A-|----|-B-|-C-|----|-D-|----|-E-|-F-|----|-G-|----|-A-|----| 
D ||----|-E-|-F-|----|-X-|----|-A-|----|-B-|-C-|----|-D-|----|-E-|-F-| 
A ||----|-B-|-C-|----|-X-|----|-X-|-F-|----|-G-|----|-A-|----|-B-|-C-| 
E ||-F-|----|-G-|----|-X-|----|-B-|-X-|----|-D-|----|-E-|-F-|----|-G-| 

Here it is in tablature form:

E||--------------------|--------------------|------------5----8-||
B||--------------------|--------------------|--5----8-----------||
G||--------------------|------------5----7--|-------------------||
D||--------------------|--5----7------------|-------------------||
A||------------5----7--|--------------------|-------------------||
E||--5----8------------|--------------------|-------------------||


This is the A minor Pentatonic scale. Of course, you can play this anywhere on the fretboard as long as the A,C,D,E and G notes are used. 

Here is the A minor Pentatonic scale, starting on C:

E||---------------------|------------------|------------8----10-||
B||---------------------|------------------|--8---10------------||
G||---------------------|------------7--9--|--------------------||
D||---------------------|--7---10----------|--------------------||
A||-------------7---10--|------------------|--------------------||
E||--8----10------------|------------------|--------------------||


Here is the A minor Pentatonic scale, starting on D:

E||--------------------------|-------------------------|
B||--------------------------|-------------------------|
G||--------------------------|--------------9----12----|
D||--------------------------|--10----12---------------|
A||--------------10----12----|-------------------------|
E||--10----12----------------|-------------------------|



--------------10----12----||
--10----13----------------||
--------------------------||
--------------------------||
--------------------------||
--------------------------||


Here is the A minor Pentatonic scale, starting on E:

E||--------------------|--------------------|------------0----3--||
B||--------------------|--------------------|--1----3------------||
G||--------------------|------------0----2--|--------------------||
D||--------------------|--0----2------------|--------------------||
A||------------0----3--|--------------------|--------------------||
E||--0----3------------|--------------------|--------------------||

Here is the A Minor Pentatonic scale, starting on G:

E||--------------------|--------------------|------------3----5--||
B||--------------------|--------------------|--3----5------------||
G||--------------------|------------2----5--|--------------------||
D||--------------------|--2----5------------|--------------------||
A||------------3----5--|--------------------|--------------------||
E||--3----5------------|--------------------|--------------------||

You can form other minor Pentatonic scales the same way in any key. For example, you can play the E minor Pentatonic. Just use the above formula against the E Major Scale. If we do, we get the notes E G A B D. 

Here is an example of an E minor Pentatonic scale. 

E||--------------------|--------------------|------------0----3--||
B||--------------------|--------------------|--0----3------------||
G||--------------------|------------0----2--|--------------------||
D||--------------------|--0----2------------|--------------------||
A||------------0----2--|--------------------|--------------------||
E||--0----3------------|--------------------|--------------------||


Practice playing the E minor Pentatonic scale, starting with other notes in the scale, like we did with the A minor Pentatonic scale above. 

The minor Pentatonic is a good scale to work on your solo techniques. Although it is simple and a good scale for beginners, it is used by even the best guitarists in their solos. The Using Scales lesson has some simple riffs using the minor Pentatonic scale.