I'm not an American, but I know that the number 101, often used postpositively, is used to mean fundamentals/rudiments of a particular scholarly subject. I know (partially as a hunch but I also looked it up) this is because introductory courses in American, or maybe North American college are given the number 101.

But now I want to know a bit more about this numbering system, because I was watching a video called: Chemistry 107. Inorganic Chemistry, and I wondered exactly how far along or advanced this is? Is it guaranteed that there are 5 other courses between 101 and 107 that are intermediate between these?

The only information I got from Wikipedia was that:

This common numbering system was designed to make transfer between colleges easier.

So if anyone has gone through an education of this numbering system, could you explain roughly how this is arranged? Is Chemistry 107 really far ahead, and does it mean that there are 5 previous courses before it?

And here is the video in case it helps in answering my question.

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    I'm not from the US, but from what I understand, the first number is basically the year and the rest is the number of the course. 101 is the most basic course in the first year, 102 would be in the first year but for someone who's already taken the subject in high school, etc. 107 wouldn't be that advanced as it's still a first-level course.
    – user9646
    Commented Apr 27, 2018 at 10:16
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    In my dept., 100-level courses were introductory, 200s were for non-majors, there were no 300s, and the 400s were all the major courses. It's pretty variable Commented Apr 27, 2018 at 17:53
  • A remark that doesn't warrant a full answer: some schools, such as community colleges in Texas, use a four-digit system, i.e., MATH XXXX, where the first digit is used to indicate the level akin to 100, 200, etc. and the second describes the course's worth in credits. As an example, MATH 3325 is a junior level, three-credit course. Commented Apr 27, 2018 at 19:24
  • When I was at Univ. of North Carolina in the late 1970s, precalculus was Math 30, Calculus I-IV was Math 31-34, etc. Graduates only courses (actually means you have to get permission if you're an undergraduate) started at 200. However, I just looked and they don't use the same numbering system now. But Harvard still has a similar numbering system in place. For example, Math 55 at Harvard would probably not be considered an introductory course! Also Stanford University -- see their Math 61CM, 62CM, 63CM courses. Commented Apr 27, 2018 at 20:43
  • My school uses 100 for absolutely introductory courses, 101 /102 if it splits the 100 course into two and expands the content (101 then becoming the introduction). (I've also seen it turn ECON 101 & 102 into ECON 100 + 201 + 202, so there are some permutations.) Higher 10x courses would be for slightly more advanced content or slight variations, e.g. we use MATH 103 for an introductory calculus course specifically aimed at business and social science students, but MATH 101/102 for the math major stream. Commented Apr 27, 2018 at 20:48

6 Answers 6


A common system works roughly as Najib Idrissi describes: courses numbered 100-199 are first-year courses, which either have no prerequisites or only high school-level prerequisites. Courses numbered 200-299 are second-year courses, which have 100-level prerequisites, and so on.

But this system is by no means universal in the US, nor does the rough description above capture all aspects of it accurately. The "and so on" above would lead you to guess that 400-499 are the most advanced undergraduate courses, and 500+ are graduate courses. And that's the case at some universities. But at my current institution, 300-399 designate the most advanced courses intended primarily for undergraduates (including those in their fourth year), and courses starting at 400 are graduate classes. At another institution I've been at, "lower-level" undergraduate courses are 1-99, "upper-level" graduate courses are 100-199, "lower-level" graduate courses are 200-299, and "upper-level" graduate courses are 300-399. And in any of those systems, you can't always guess from the number alone the relationship between Basket Weaving 125 and Basket Weaving 147.

To address your question about the online chemistry course, you can't even assume that Chemistry 101-106 even exist at the institution in question; if they do, they may or may not have any clear relation with 107. When I was an undergraduate, there were two distinct introductory chemistry sequences for different audiences: 105-106 and 107-108. So there was no 101, and 106 was a more advanced course than 107.

So if you want to know the place of a specific course in the curriculum, you have to consult the course offerings of the specific institution.

tl;dr (summarizing this and other answers, and many more details in comments): There is no one system for course numbers, even at a given institution. At best you can make a rough guess about a course number's meaning. If you really want to know, you need to find information from the specific department.

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    Sometimes, "100" is a course intended for non-specialists of the subject. So "Psychology 100" may be for Arts students (Literature, History) who need a 1st-year Science credit. For students planning to proceed in Psych, there would be a 101 or others. At one large University, as I recall, Physics 100 was a science credit for Arts students, and there were other 10x courses specifically intended for majors in Engineering, Medicine, Pharmacy, and others--each separate, I think, so as to be scheduled compatibly with the courses of those specialties. Actual Physics majors took Physics 130.
    – CCTO
    Commented Apr 27, 2018 at 18:02
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    And at the low end of things, 0-level courses are typically remedial, non-credit courses covering things the student didn't learn in high school but should have, eg. "Math 050: Trigonometry".
    – Mark
    Commented Apr 27, 2018 at 20:57
  • @Mark That is not a "typical" system. Some schools use numbers below 100 that way, but many don't. Again, while the first digit often has significance, which numbers mean what varies a lot.
    – Henry
    Commented Apr 27, 2018 at 21:12
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    Sometimes, 101 existed in the past, but got split or combined with another course, or various other things, and the number never got reused.
    – Riking
    Commented Apr 27, 2018 at 22:17
  • @CCTO And for another example, mine was the opposite - CS 100 was the overview course for CS majors. CS 105/106 were the intro-to-programming courses for non-majors, and CS 115/116 were the same intro courses but geared towards majors. (And to this answer's point about gaps, there were no other 100-level CS courses)
    – Izkata
    Commented Apr 28, 2018 at 4:14

The numbering system isn't nearly that consistent across American universities. "Subject 101" isn't really the introductory course in Subject at most schools.

Based on my experiences on a few schools, here are the consistent patterns I'm aware of:

  • Course numbers are typically three digit numbers
  • The first digit does typically indicate the level of the course, with 1XX courses being lower level than 2XX courses and so on, but the significance of the first digit can vary wildly (4XX courses could be undergrad courses or upper level grad courses at different schools, for instance).

It's impossible to guess at the significance of the last two digits of a course number without knowing the specific courses involved. Some of the principles that lead to choosing specific numbers are:

  • Sometimes consecutive courses do get numbered consecutively, so 130 and 131 might form a related sequence. Conversely, my experience is that when courses don't form a natural sequence, they rarely (but not never) get consecutive numbers, to avoid confusion: there are always many gaps in the numbering system.
  • Sometimes the second digit has significance - it might be that courses whose second number is a 4, regardless of level, are all inorganic chemisty, so 141 is the first inorganic chemistry course while 440 is the number of a graduate seminar in inorganic chemistry.
  • Often when a course is removed or dramatically changed, its number will be retired for a time: it would be confusing if 107 meant very different things for people graduating from the same school in the same year because they took the course in two different years.
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    Another principle that is sometimes used: odd numbers for the fall semester courses, even numbers for the spring semester courses. Commented Apr 27, 2018 at 16:14
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    Re 4XX courses, what I've seen is that the same course will be open to grad or undergrads. Undergrads sign up for 4XX, grads for 6XX - same course, same instructor, perhaps a few extra assignments required of 6XX. Then 7XX courses are generally grad-level only.
    – jamesqf
    Commented Apr 27, 2018 at 17:35
  • @MichaelSeifert Or vice versa. I've seen the evens for sone semester and odds for another pretty frequently, but I haven't seen one be more common than the other.
    – RothX
    Commented Apr 27, 2018 at 17:52
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    @jamesqf The pattern of which initial number corresponds to which level varies a lot by institution. I've never been at a school which routinely used 7XX numbers, but I can certainly believe they exist.
    – Henry
    Commented Apr 27, 2018 at 18:59
  • My school's grad courses are 8XX, so I think this is very school-dependent. Commented Apr 27, 2018 at 20:49

When people in the U.S. talk about a generic introductory course, say "Underwater Basket Weaving 101", we often give it the number 101.

This is just a linguistic shorthand; introductory courses are labeled 101 at relatively few colleges and universities. The actual numbers depend on the university, and the systems vary wildly, and can even vary somewhat between different departments at the same colleges.

For example, the introductory courses in mathematics at various universities (calculus or precalculus) are labeled:

M.I.T.: 18.01 (18 is the math department number).
Princeton: MAT 100, followed by MAT 103.
Harvard: Math 1a.
Williams College: Mathematics 130,
Purdue University: MA 16500 or MA 18100 (the second is honors calculus).
Mount Holyoke College: Math 101.

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    It's worth adding that there are often multiple introductory courses for a subject like mathematics, usually intended for different students with different needs. There might be an intro course for students who intend to become math majors, one for students who will be studying engineering, one for students studying economics/social sciences/life sciences, etc... Each course will have different numbers. That system will also vary wildly depending on the department. Commented Apr 27, 2018 at 19:55
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    This nicely summarizes what I began to read from the other answers: "101" is mostly used only in this proverbial sense. In which case it need not even apply to actual courses, but any profession ("we found the suspect, that was just police work 101") or other activity ("How can you wear socks and sandals? That's fashion 101"). E.g., a possibly good and idiomatic translation into German might be "Das kleine Einmaleins des Unterwasser-Korbflechtens" (literally, "the rules of multiplying single-digit numbers in underwater basket weaving") Commented Apr 28, 2018 at 8:47

As others have said, there's a lot of variation in how courses are numbered at different universities. One example that's somewhat different from those that have already been mentioned is the University of Texas at Austin.

UT Austin uses the first digit to indicate the number of credit hours that are awarded for the class (which is usually roughly equal to the number of class hours per week.) The remaining two digits indicate the relative level of the class: lower division (freshman/sophomore), upper division (junior/senior), or graduate.

For example: The first class taken as part of a bachelor's degree in physics is PHY 301 (Mechanics), which is also open to other majors. It requires prior or simultaneous enrollment in PHY 101L, which is a laboratory course. There's also PHY 104 (Introductory Physics Seminar) which is a high-level overview of the field, and is roughly equivalent to what one might expect from a "101" class.


Short version:

  • Only the first number matters in course descriptions.
  • 101 courses are special in that they are designed for anyone at the university to take them, and have no prerequisites.

100-level courses (sometimes called 1000-level courses) are designed for all students, regardless of major or college*.

  • So Econ 201, Econ 220, and Econ 2051 are meant for Econ majors only. These courses presume familiarity with the department's field. In other words you'll be at a serious disadvantage if you haven't taken a few 100-level courses.

200-level courses (sometimes called 2000-level courses) are designed explicitly for majors.

  • So Econ 202 and Econ 240 are designed for econ majors. Thus most students will be econ majors, and the course will assume you have some general background knowledge of economics. Ideally from taking 100/1000 level courses.

  • In most cases 200-level and up courses are not going to be designed (or fair) for cross-school (cross-college) students. In other words, the pre-reqs for 200-level and up courses are "you need to have had at least a year of education in this school in order to know what's going on in class".

300-level and up (or 3000-level and up) courses and up vary widely by school and department/major.

  • Where I went to undergrad, 300-level sometimes meant you had to have taken at least 3 200-level courses in the department.
  • Sometimes 400-level means seminar, or masters students only. Sometimes 300-level means double-length courses.
  • If your school has 500-level courses, these are normally grad student-only courses, though.

* if your university has multiple colleges. For example, Cornell has an engineering college, a fine arts college, a liberal arts college, an industrial and labor relations college, a hotel management college, an undergrad business school, and more. Here's a better explanation of this.

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    I don't think the practices described here are remotely universal. For instance, some schools may use the distinction between 100 level and 200 level to distinguish between non-major and major courses, but many other schools do not. Nor is it consistently true that 500 level courses are grad student-only.
    – Henry
    Commented Apr 27, 2018 at 19:02

A university can use any system they want. Especially older and prestigious universities will have weird systems, since they are less concerned about being accepted as "legitimate" (their legitimacy is already beyond question).

The "100-system" is pretty common. Usually the first number is the year in which students are expected to take it, and the second number is the semester. But this system often creates problems:

  • Sometimes, Math 101 is taken in year 1 by some majors but year 2 by others.
  • Sometimes students from a major can choose from, eg, organic or inorganic chemistry for their third semester. So which one gets to be called Chem 201? Regardless, the other one will be called something else.
  • Sometimes a class can be taken at any time as an elective, so has no "default" time it is supposed to be taken.
  • Sometimes there will be multiple versions of a class, but later on the 101 course will be discontinued, while the other remains.

The only real rule is that if a course has a certain number, no other course will have that number.

However, "Something 101" has been used for a time in colloquial English to mean "the basics of", "an introduction to" or "a crash course in" something. Funnily enough, this has resulted in me taking some courses which claimed to be the "101" of something, but were advanced, graduate-level classes (the ones I took did not actually have a course code of 101, though).

So the answer is, there isn't really a system, and when people refer to "101", they probably mean it as a figurative expression, and not an actual system.

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