A test score marked “on the curve” means that the marks will roughly follow what is often called a Gaussian Probability Distribution—or, more commonly, a bell curve. This symmetrically shaped curve is based on the test scores of the exam. In a perfect world, one-sixth of the scores would be on either end of the curve, with more than two-thirds falling in the middle, creating a normal distribution.

But most test results are not ideal. Thus, when a plot of the number of students versus the marks received are viewed, the relative difficulty of the test is known. It is then up to the teacher to decide how to distribute the grades (this is usually done by comparing each student’s mark to the distribution curve). The teacher then decides where to cut off the passing and failing marks, which is often referred to as curving the grades. (For more about normal distribution, see “Applied Mathematics.”)