Take the course code PHYS 3054 or PHYS 354. The physics department doesn't offer 54 courses at the 300 or 3000 level.

I imagine some universities subscribe to a set of common best practices for course numbering. So I suspect that there might be an answer that applies to the numbering systems of a number of institutions.

  • At times middle digits may correspond to difficult, eg PHYS 3054 is followed by PHYS 3151, but both are "3000 level" courses. They may also be thematic, for example PHYS 47xx courses may all be about magnetism, while PHYS 48xx courses may all be about optics.
    – chmullig
    Commented Apr 24, 2014 at 3:50
  • 3
    In practice, they most likely represent which numbers were available when the course was proposed.
    – JeffE
    Commented Apr 24, 2014 at 7:03
  • 2
    "I imagine some universities subscribe to a set of common best practices for course numbering." I've never seen any evidence of that. Commented Apr 24, 2014 at 11:25

2 Answers 2


I suspect it really depends on the university. But in my university, it represents the subfield in the department within which the course is offered.

For example (I'm in electrical engineering), EE X1X might be courses on power systems engineering, EE X2X on communication networks, EE X3X on signal processing, EE X4X on electronics...

But in my department at least, there are enough misnumbered courses (for "historical reasons") to confound anyone who doesn't know it and is looking for a pattern.


ff524 mentions two of the major factors---subfield identification and historical reasons. Even without a rigorous system of subfield identification, there's typically a lot of spacing in course numbers so that unrelated courses don't typically have adjacent numbers (and indeed, there's plenty of space between unrelated courses).

This goes along with numbering sequences sequentially---520 and 521 might form a two course sequence, in which case you don't use 522 or 523 because someone might get confused and think they're part of the sequence. And then later, when you retire 520 and 521 and combine them into a single one semester course, you might call it 522, to remind people it's similar, but you don't use the same number so people can easily tell which course someone took. This leaves a lot of historical detritus (you want to wait a long time before ever using 520 for something new), which leads to lots of widely spaced numbers.

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