FamousThe Twilight Zone1960 · Marius Constant
Main title: the opening guitar riff, taken from Constant's CBS library cue 'Étrange No. 3'
Two guitars play the riff: one repeats a high B♭ while the other circles A, G♯ and E just underneath it. Against the B♭ those notes make a half step, a whole step and a tritone in turn, three of the most jarring intervals there are, which is why it sounds so uncanny. The published guitar arrangement names the overall harmony C7♯5, though the riff itself has no C.
Also called Twilight Zone, Twilight Zone theme, Étrange No. 3, C7♯5 (as the published sheet music has it)
Sources: Bill Wrobel, The Twilight Zone rundown ('Main Title I', hand-copied from Constant's score in the CBS Collection at UCLA) · Hal Leonard/Musicnotes 'Twilight Zone Main Title', easy guitar tab (page 1 preview) · Wikipedia, 'Marius Constant' · Classic Themes, 'The Twilight Zone'