Caption: The Gaussian distribution (AKA the normal distribution, AKA the bell curve).
Features:
There are good reasons why it has wide application---but we won't go into that here.
For the Gaussian distribution, ∼ 68 % of the population is within one σ of the mean, ∼ 95 % of the population is within 2σ of the mean, and ∼ 99.7 % of the population is within 3σ of the mean.
How is this done?
The instructor calculates the μ and σ for the actual class. Then for example, the instructor decides that he/she wants about 16 % of the class to have A's. From the curve in the diagram, the instructor knows that about 16 % of the Gaussian distribution, there won't be exactly 16 % A's assigned, but usually the number will be close to 16 %.