Wikipedia defines 'grading on a curve' as a
a statistical method of assigning grades designed to yield a pre-determined distribution of grades among the students in a class.
The article does not say much about when it's appropriate/inappropriate to grade on a curve, but it does state
The ultimate objective of grading curves is to minimize or eliminate the influence of variation between different instructors of the same course
I take it then that if a course has only one instructor then that would remove an argument for grading on a curve. It also seems to me that it might be inappropriate to grade on a curve when a course cohort is extremely small, since it would then be hard to say what grades 'should' be given.
What other factors make grading on a curve more/less appropriate? Are there factors that make it absolutely inappropriate?