Yale College has an inventory of thousands of courses. Departments sometimes find they are running short on available course numbers to assign to new courses.

As a practice, it is a good idea to clean up your inventory every few years; it is a good summer project. Deactivate courses that haven't been taught in seven years and courses taught by faculty who have retired or left Yale. 

Email registrar@yale.edu to ask for a report that lists courses that have not been taught in 7 years.

See How to Deactivate a Course.