Why do U.S. school grades go A, B, C, D, and F? Why not "E"?

Robert Frost:

The modern letter grade system began at Mount Holyoke College in Massachusetts in 1897. The system was picked up by other schools and within about 20 years had become the norm across America. During this same period, immigration and mandatory attendance laws were resulting in more crowded schools and a higher student-to-teacher ratio. The letter system caught on with teachers because its simplicity made their job simpler as they became responsible for grading more students. Also at this time, it became more of a standard to report grades to parents, so a scale that was easy for parents to understand was desirable.

Mount Holyoke defined their original scale as follows:

A: Excellent, equivalent to 95— to 100 percent

B: Good, equivalent to 85— to 94 percent

C: Fair, equivalent to 76 to 84 percent

D: Passed, equivalent to 75 percent

E: Failed, less than 75 percent

E was quickly replaced with F, because "F for failed" was more intuitive than "E for ... excellent or failed?"

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