The standard z-score. But the underlying concept is pretty straightforward. Let's say I had a probability distribution and I get some x value that's out here and it's 3 and 1 half standard deviations away from the mean. Then it's standard z-score is 3 and 1 half. But anyway, let's focus on the purpose of this video. So that's what the normal distribution, I guess the probability density function for the normal distribution looks like. But how did it get there?