Using the seven note natural minor scale as a starting point, we remove two notes to create the five note of the minor pentatonic.

Again, the notes removed are those which are a half step from the chord tones of the 'one' chord in the given key.

For example, taking the A minor scale, an Am chord was created from the 1st, 3rd and 5th notes of the scale:

A B C D E F G A

The two notes a half step from the chord tones (namely B and F) are removed to avoid any potential for clashing, dissonant notes in a melody over the Am chord.

The notes of the A minor pentatonic are therefore:

A C D E G

If we identify those notes in intervals by their scale degree in relation to the tonic (A) we will have :

1 ♭3 4 5 ♭7

An E minor pentatonic scale would therefore be:

E G A B D

A B minor pentatonic scale would be:

B D E F♯ A

The formula for the minor pentatonic horizontally is therefore:

Minor 3rd-Whole-Whole-Minor 3rd-Whole

b3-W-W-b3-W