Fall-term courses are indicated by the letter “a” (e.g., ENGL 201a), and spring-term courses by the letter “b” (e.g., ENGL 450b). Departments number their courses according to different schemes, and course numbers do not necessarily reflect whether courses are more or less advanced or more or less difficult.
In some departments, courses are numbered in ascending order, going from introductory to intermediate and advanced. In others, courses are grouped primarily by subject matter, with the result that the number of a course (usually with the exception of the 400 range) bears no particular relation to its level of difficulty.
More precise indications about the level of a course may be found in the introduction to some department’s entries in the Yale College Programs of Study.