[llllllllllll]

The rules you've consciously learned about language constitute a tiny fraction of the rules that govern our use of language. Many of them ("don't split infinitives!", "no prepositions at the end of a sentence!") fail to be descriptive.

Not that it matters, but "biweekly", "biannually", and "bimonthly" have always only had a single meaning for me. I've heard the debate about them, but I don't think I've ever actually seen them used to mean "once every two weeks/months/years".

[mrxd]

Drawing grammar rules for English from Latin might be a bad idea, but it tells us nothing about the value of rules as such.

Every dictionary gives multiple definitions of biweekly, etc. To professional linguists, this is an example of the wonderful flowering of the diversity of language or whatever and is probably much more exciting to study. Everyone else just cares about when to pick up their paycheck.

[llllllllllll]

My point is that if you think the rules we learn in school about language are the same rules that allow us to communicate and understand each other, you're failing to realize the massive complexity of language.

As for those "bi-" words, there seems to be more consistency with "biannually", than with "biweekly", or "bimonthly". Oxford only gives a single definition for "biannual":

http://www.oxforddictionaries.com/definition/english/biannua...

As a professional linguist, there are millions of questions about language I find more exciting than this :)