Unit codes

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For each topic in OTIS, one or more versions can be offered. The versions are identified by three-letter codes (like BFW or ZAX), which are here for historical reasons, and it's too late to change them.

These codes are typically cryptic to outsiders, but here's an explanation…

First letter: Difficulty track

Roughly, the three tracks are:

• B is the easiest (problems range from sub-IMO level to IMO2),
• D is medium (problems span IMO range),
• Z is harder (problems span IMO2 - IMO3).

There is some grey room in this department.

Second letter: Subject

For example A is algebra, etc. These are the letters appearing on the index page.

Third letter: VErsion identifier

The third letter is a version identifier (either W, X, or Y). Some topics have multiple versions so they can be repeated in different years.

The choice of which letter is used is essentially random. It's not meant that one version is "first" or "harder", etc.