If you take i and square it, you get a -1. Let’s go over that again: i*i = -1. As that’s not possible, it’s imaginary, which is why it’s represented by the letter i. However, in electrical engineering, "i" represents current, so we use the letter "j". In a graph, we represent the real numbers on the x-axis and the imaginary numbers on the y-axis, and when calculating impedance, imaginary numbers are a great way to show the varying resistance at different frequencies.