It's typically the 80-90% range. It's also typically divided into thirds for B-/B/B+. I actually got a B+, so it would have been closer to 90%.

Many places in the US, each grade covers 10%, so As (with plus or minus) go between 90 and 100, etc, with D from 60-70 and F below 60.

Things can get a bit customized at either end of the spectrum, but generally each letter covers 15%, with A..D covering the span 100% to 40%.

So a B would be 70 to 85.

Is this a new thing? I graduated highschool in 2001 and each grade letter was 10% and equated to a grade point.

    90%+ => A (4.0)
    80%+ => B (3.0)
    70%+ => C (2.0)
    60%+ => D or F depending (1.0 or 0.0)

At university, we only used grade points and they were very, very similar in scale.

It was the grading system in Ireland when I graduated secondary school. However, it looks like the meanings of the letter bands varies dramatically between countries.

70 - 85 for a B?! I wish I had that deal.

At my university the letter grades were

    A+  99.00 - 100.00%
    A   93.00 -  98.99%
    A-  90.00 -  92.99%
    B+  87.00 -  89.99%
    B   83.00 -  86.99%
    B-  80.00 -  82.99%
    C+  77.00 -  79.99%
    C   73.00 -  76.99%

Shouldn't that sort of grading be on a bell curve? That would seem to be the fairest.

Many smaller (<60 students) math classes, in my experience, have bimodal distributions of scores, so a bell curve by definition simply doesn't make sense. In addition, there's been a move to standards-based or mastery-based grading, that is, grading according to what you know of the material rather than how you compare to your neighbor. This allows comparisons over time and consistency with regard to subsequent classes -- if you have a C as a prerequisite for the next class, then a C should indicate the same mastery of material rather than the same relative position in the class.

Let's say that most math majors take this course in the first semester of their sophomore year. Then the bell curve grading gives you a much lower grade for identical work if you take the class in the first semester vs second semester. You also get a lower grade for going to a better school. You even get a lower grade for helping your classmates.

This makes the grades unfair and not useful for judging mastery of the subject. It only makes sense if the goal of a course is to beat the other students. For most courses, the goal should be learning.

Based on my memory of what the teacher said in my statistics class in school, exam results in Scotland are normalized as z-scores vs that year's population. At least they were a zillion years ago.