Why Are Introductory Classes Called "101"?

From www.mentalfloss.com:

The first recorded use of an introductory class being designated as "101" was in a University of Buffalo course catalog dated 1929. It wasn’t until the early 1930s that universities in the United States started using a three-digit system to identify their courses. The method wasn’t quite uniform, but it was more logical than the non-system of naming courses that had previously been in place.

In the 1930s, college students started regarding a university degree as a means to a better job, and as a result, universities started added more specialized classes to their curriculum. Students were also traveling further afield after graduation in search of work, so it became important for a potential employer to be able to compare candidates: Was a passing grade in Cost Accounting 203 at Kent State the same as one in Business Accounting 4 at the University of Michigan?

Eventually, colleges started using a three-digit designation, in which the first digit indicated the academic level (1=Freshman, 2=Sophomore, etc.). The second digit usually represented a department (English, Science, etc.) and the third the level of the class within the department. These were not hard and fast rules, and still vary from school to school.

However, as the three-digit system became more commonplace, it seemed that "101" always represented a basic beginning course, no matter what the discipline. By the late 1960s, the phrase was starting to enter the vernacular at large, outside of the collegiate realm.

Yet another example: Technical Note NAND Flash 101: An Introduction to NAND Flash and How to Design It In to Your Next Product

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