At various points in my teaching career, I've heard of classes in which students were told something like this the first day: "Everyone has an A in this class unless you don't do the work" or "An A in this class is yours to lose" or "If you complete these 5 assignments under our contract, you will get an A." The idea is that this would dispel the students' anxiety about the class and make them work harder for the sheer joy of learning.
I could see the traces of this kind of grading when students would get to my class, look at an essay exam or a paper that I'd handed back, and say, "Why didn't this get an A? Where did I lose the points?"
My practice, as I would always explain to the students when the first exam was handed back, has always been the opposite: a paper starts from zero points/grades and rises up the grading scale based on its quality. A B paper isn't an A paper gone bad in some point-driven way but a paper that began as a 0 and worked its way up to a B ( or"good," as students often forget) level. Better papers worked their way up to an A. Some papers worked their way up to a C.
It seemed to me that the "A is yours to lose" theory of grading would create more anxiety than it would solve, since the only way you could go in such a system was down. Every evaluation opportunity becomes a chance only to fail or to maintain the status quo rather than improve. The best you can do is break even and not lose, but you never really win.
I was thinking of this recently because of something a student wrote about this summer. Her assumption was that all teachers graded on the "points down from an A" model, and her suggestion was that teachers instead start from the bottom and grade upwards since that is more motivating and since that is what students are used to in every game they ever play on their iPhones. I hadn't thought of grading and motivation in terms of games, but it's a great metaphor.
What's your practice?