Maths? Math? Math's? Debate sparked in /r/iamverysmart

I think you misunderstand what "prescriptive" means. It doesn't mean that there is a single, unitary rule the governs some sort of platonic English, and every example of English is a derivation of this. Prescriptivism doesn't mean that the "prescriptive" form is derived from some set of rules (often it isn't - English is full of seemingly arbitrary prescriptive rules). It sort of seems that is what you are saying.

Linguistic prescriptivism means, simply, that someone somewhere has said "this is the correct form". It doesn't preclude someone, somewhere else, saying "this is the correct form". And prescriptive rules in English can vary based on location and culture, because English varies based on location and culture.

So if we were to look at Australian English versus American English, we might note that most Australians abbreviate "mathematics" as "maths", whereas most Americans abbreviate it as "math". This is a descriptive approach - we have looked at actual usage.

The prescriptive approach is found by looking at authoritative sources. So in America, you'll find dictionaries (so far as they are prescriptive), teachers and style manuals saying the correct form is "math". And I'm sure you'll find a small number saying "maths" as well. BOTH of these claims are prescriptive claims, regardless of how they derive them or the fact that they contradict one another. A prescriptive claim is simply someone saying "this is how it SHOULD be".

In Australia it is the opposite - all of our prescriptive authorities that I checked prescribe "maths" as the proper abbreviation.

These are all, unambiguously, prescriptive claims. They DO vary with culture and location, they apply different logic, and they are all "equally" correct, notwithstanding that some prescriptivists seem to think that their form is the "one properly derived TRUE form". You seem to have fallen into precisely this mistake - once a claim is made that one form is the correct one, that is a prescriptive claim, yet in this case there is no reason to consider "math" the correct prescriptive form as opposed to "maths".

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