In the past, the first number in the course number indicated a variety of different things, including challenge level. As you can see below, that is no longer true as we have attempted to create greater consistency in our course numbering patterns:
First digit: year in a sequence
1XX = Course for first-year students in the discipline and/or for preps, e.g. PY150Y.
2XX = Course for second-year students in the discipline and/or for lower mids.
3XX = Course for third-year students in the discipline and/or for upper mids. Introductory-level courses that are open only for upper mids or seniors are also marked at the 300-level. Examples of this might be the upper-class version of Physics (PY350Y) and U.S. History for new upper mids and seniors (HI350Y).
4XX = Course for fourth-year students in the discipline and/or for seniors. Departments like H&SS that mainly have one level of course offerings for both upper mids and seniors may mark all upper-level electives at the 400-level.
Additional hundreds designation (for Languages and Math only):
5XX = Course for fifth-year students in the discipline
6XX = Course for fifth-year students in the discipline
Second digit: honors vs. non-honors
X0X-X7X = The majority of classes are marked here—there is generally no distinction in the challenge level within this range, though a 0, 1, or 2 may indicate that the course is designed to be more approachable.
X8X-X9X = Honors-level courses (note one exception is independent studies, which are X99, but not honors courses).
Third digit: course identifier
No specific meaning. These are now available to expand the number of electives that are possible to keep in the course catalog.
Letter designation: course timing
F = Fall
S = Spring
P = Portals (Summer)
Y = Year-long