Depends on the grading system and calculation method. A traditional percentage based system is done by 10 point increments:
A: 90-100
B: 80-89
C: 70-79...
That is generally considered standard with the plusses and minuses given in the two points adjacent to the next corresponding grade.
Of course, the above system can be adjusted in countless ways. If I were to give a high school class a middle school level assignment (I don't know why I would... but if I did), it might be appropriate to adjust the scale upward accordingly. Or downward in the case of a particularly rigorous assignment.
Another approach is to curve grades based on actual performance. There are many ways to do this, including
a) Using the top attained score rather than the total number of problems as the basis for calculating percentages (nobody got 100/100, so I'm calculating this on the basis of 94 which was the top attained score).
b) Eliminating commonly missed problems.
c) Using a standardized distribution such as a bell curve, which places the bulk of students in C range and radiating outward by some quantile in both directions (this used to be much more common, I rarely see this done anymore).
d) I'm going to stop, there are thousands of ways to do this... you get the idea.
BUT, other options and systems obviously exist...
In the International Baccalaureate Program, grades are given on a scale of 1–7, with 7 representing highest achievement. But, because these grades correspond with rubric requirements, one could theoretically answer 90% of questions on an exam correctly, but still get a 5 if the missed 10% represented crucial problems that corresponded to the 6 and 7 grade bands.
Percentile grading is still used at some universities, which is similar to curving grades, but results in the top 20% getting the equivalent of an A, the next 20% a B, and so on.
All of this, by the way, is one of the justifications for standardized testing. At any given school, any number of these systems can be employed in different classrooms. Administrators often try to crack down on really radical variance, and increasingly campus wide grading software is being used (like Blackbaud), which enforces a 10% traditional grade scale.
