Based on the standard tuning of a guitar E, A, D, G, B, E and assuming you use E as your tonic, the scale I, 2, 3-flat, 4, 5, 6, 7-flat which translates to E, F#, G, A, B, C#, D should be a base scale. If you strum a guitar open, all the strings fall within this scale. It does have a flat seventh and a corresponding flat third.

If you take out the 2 and the 6, this becomes the Minor Pentatonic Scale.