Suppose that a set of test scores from a class you teach is symmetric and bell-shaped. That's means that there are some scores significantly below the mean and some significantly above the mean, so that it would be inappropriate to "give everyone an 'A'," for example. What z-scores would you use as cut-offs for A, B, C, D, and F?
That would vary from teacher to teacher, but here would be an approximation of mine.
A > Z = +2
+1 < B < +2
-1 < C < +1
-1 < D < -2
F < -2