The B paper shows greater complexity of thought and development, while sustaining clarity in expression. It has few or none of the common errors in the use of conventional written English. The A paper is outstanding work. It is clearly a superior performance according to the criteria of clarity of expression and logical development of a central idea. It shows originality of thought and imaginative competence in the development of the material.
D indicates unsatisfactory performance.
In particular, many students assume that professors outside of the English department grade for content alone, not grammar or writing style. If you do grade down for poor writing, tell your students from the outset. Upholding high standards is one of the best ways to encourage students to focus on their writing skills. You might even have students make rank order evaluations of essay responses, even develop a set of essay scales, or you can provide your own. Harper Collins, 1992.