What are the rules for demoting proper words to generic words?
I recently ran across a blog post that referenced Spartan meals. I would be inclined to use Spartan if the foods were common in Greece, but lowercase the s to indicate the servings were spare. Similarly. I'd use French fried potatoes only if they were actually fried potatoes that were French, not merely Idaho Russets fried in deep oil.
The Big Dic says "Spartan"is sometimes capitalized when it means meager. Does the AP Stylebook, or the MLA or some other commonly received authority have guidelines for when a proper word is to be demoted? (I don't think I have ever seen someone being "Gypped", but I don't think that has anything to do with the Romany not actually being from Egypt.)
There was a thread about this topic I wanted to link you to, but the search engine seems to be broken. Keeps giving me an "invalid database query" error. Maybe it's that new web-hosting service Grant warned us about earlier? Is this happening to anyone else? If so, we need to let Grant know.
Anyway, that thread was about when to capitalize "Earth" "Sun" "Moon" etc. There was a pretty solid consensus on that topic. For example: We are from Earth (obviously refers to the proper noun) vs. I found this buried under 6 inches of earth (refers to the common noun).
I tend to agree with your interpretation of "Spartan" vs. "spartan" for the same reasons. Especially since the former refers to a culture, and the latter is used as an adjective. Just like Equator and equatorial.
Yes, both style guides and dictionaries will note "often not capitalized" or "sometimes capitalized" for the common noun derivative of a proper noun. The evolution from proper noun to common noun is largely a matter of common usage. One classic example is Kleenex, originally a brand name, devolving to mean any facial tissue.
About the only "universal guideline" I've ever encountered is: When the noun names a specific or unique person place or thing, you capitalize it. This works well for West vs. west, Red River vs. red river, Moon (Earth's) vs. moon (a body orbiting a planet), North Pole (of Earth) vs. north pole (of a magnet). But this begs the question of "Just how specific does it have to be?"
Note regarding one niche area: SI units. Most SI units have been given the names of scientists who worked in the relevant area. Example: the "newton" is a unit that measures force. Sir Isaac Newton defined what force meant. The unit is never capitalized (but its abbreviation "N" is). One could argue that the newton is a "specific or unique thing." But we never capitalize "pound" or "ounce" so this isn't really limited to SI.
I think, without exception (dangerous claim, I know, since I expect someone will come up with a counter-example) that any adjective derived from a proper noun will NOT be capitalized. Examples: spartan, equatorial, lunar, martian (unless there's a Martian present), midwestern, etc. There is some discussion about possible exceptions here, mainly related to eponyms. Examples: herculean, newtonian, cartesian. But they argue that, as usage expands, the capitalization is dropped.
deaconB asked: What are the rules for demoting proper words to generic words?
I'm sure there are some additional "universal rules" beyond what I suggested. There's a nice summary here. My style guide is too old to trust on an evolving matter like this. It says nothing about what to do with "iPad" or "eBay" when you have to start a sentence with those proper nouns. 
Heimhenge said
There was a thread about this topic I wanted to link you to, but the search engine seems to be broken. Keeps giving me an "invalid database query" error. Maybe it's that new web-hosting service Grant warned us about earlier? Is this happening to anyone else? If so, we need to let Grant know.
Anyway, that thread was about when to capitalize "Earth" "Sun" "Moon" etc. There was a pretty solid consensus on that topic. For example: We are from Earth (obviously refers to the proper noun) vs. I found this buried under 6 inches of earth (refers to the common noun).
Is this the link? I found it with the 'Advanced Search' even when getting the error. The regular search box has not worked for me.
EmmettRedd said: Is this the link?
No, but it's close. Good to hear it's not just me though. I'm still getting that same "Invalid Database Query" error. Even when it tells me it found a result, there's no link to take me to it. Very weird. As an Admin, will you please take care of informing Grant that the search function is broke. Many thanks.