A diminished scale is a symmetrical scale that uses alternating half and whole steps (or vice versa.)
This scale is a great choice for improvising over a diminished chord or over a dominant chord where a more dissonant "altered" sound is desired.
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(LINK) For more, check out Artur Menezes’ modern blues diminished scale lesson in his Pickup Master Class https://pickupmusic.com/diminished-scale/