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So far we've covered two different scales: the major pentatonic and the minor pentatonic. How do you know when to use which one?

Here are some general guidelines. Note that although talking specifically about pentatonic, may also be applicable to diatonic. (altho penta more forgiving, since only 5 notes, less chance a note will clash.)

How to use scales

song type major
blues A trademark sound of the blues is the minor pentatonic scale used in a major key -- C minor pentatonic for example over a blues in C major. You could also use C major pentatonic, but be careful during the IV ("four") chord; and you would never use it in a true minor key blues like "The Thrill is Gone" or "Since I've Been Loving You".
major key Rock is descended from the blues and so sometimes the minor pentatonic may still "sound right", even if the song itself is major. "Voodoo Chile", "Whole Lotta Love", "Back in Black" -- all examples of songs in the key of E major (listen for the E major chords in the rhythm guitar), but where the solos are mostly E minor pentatonic.
minor key The opposite however is not true. The major pentatonic will generally sound like dog poop if you try to use it in a minor key. (Test: try to think of a classic rock solo that falls into this category. Hint: there are none.)
pentatonic/hard to tell The third (the "mi" in "do-re-mi") determines whether a chord sounds major or minor. When it's missing, as in songs that use a lot of power chords (think metal classics like "Paranoid" and "Walk"), it can be hard to tell whether the song is in a major or minor key. You can always use the trusty minor pentatonic but be careful with the major pentatonic.