Strumming is probably the most essential guitar technique in existence. The word itself is a synonym for playing guitar, and your non-musician friends might ask you to strum a few chords for them at a party.
It’s taken for granted that any guitarist in any style should be able to do it, but the technique is actually surprisingly involved. You can spend a lifetime learning to make a simple chord progression sound good, but you’ll likely be offered a lot more gigs than someone who only practices shredding licks.
