The math majors in the room are silently nodding right now. In complex mathematics, i and j both represent the square root of -1 โ literally the same imaginary number. Engineers use j instead of i to avoid confusion with electrical current notation, while mathematicians stick with i . So yes, they're identical twins with different names, like corporate's pointless rebranding exercise that fools absolutely no one. The real difference? Which department you work in and how much you enjoy arguing about notation at faculty parties.