<p>Universities make this vague on purpose. The most you will find is some schools will post that a C or D must be explained in writing. This is so they have the leeway to review all students on a case by case basis. The best thing is to try to bring your grades up by the time you send a mid-year report. If at that point you are concerned about your grades the general thought is be proactive and contact them letting them know why your grades have slipped and what your plan is for success in the next semester. If you have a question about a specific schools tolerance for grades dropping speak with your GC who should be a good source based on past experience.</p>