Question 910986
z = 0 is at the exact center of the standard normal distribution.


Since we have perfect symmetry about the center, this means that 50% of the distribution is below z = 0. The other 50% is above z = 0.


z = 0 is an arbitrary number that was picked by convention a long time ago. Zero was chosen since it's a small easy number to work with and because you can work with positive and negative numbers (to the right and left of 0)