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Here are a few open Pentatonic Scales. I believe these are the only Pentatonic shapes the guitar allows us to play open.
As you can see with the C and Am chords, the C/Am Pentatonic Scale intersects their notes. You may have heard that everything you pick off the Pentatonic Scale will "sound right." This is why.
Here's a progression in the key of C/Am with the C/Am Pentatonic Scale.