How did this guy figure the key of the song while playing a bunch of chords over a 2-3 seconds of mu

[samsome]

starts at 2:08

How did this guy figure the key of the song while playing a bunch of chords over a 2-3 seconds of music

if i had to do it i would have to try identify chords over at least 20 seconds of music and say ok this chord is that one...this is that one...bla bla

how can he do it that way? how does that work

and how can he be sure its C minor that quick and not D# major

[WongoTheSane]

He didn't guess in 2 seconds, he looked at the video for almost a minute, and particularly the bass, which is easier to follow than the guitar. He suggests "A C F" at first (as bass notes, so those are probably the roots of the accompanying chords), but he's wrong on the A (confirmed by the guy on the left), it's a G instead. So he has C F G which is the usual i-iv-V7 minor progression. He just checks on his own guitar by playing a few chord shapes from the Cm scale: they fit, so he's got it right. Why not D#? Because it sounds more minor that major, and it's probably genre-specific too. Still impressive, I agree.