Capitalization: when does a phrase become a proper noun?

This is a question on capitalization. Proper nouns are capitalized. But how can I tell which parts of a term constitute a proper noun?

Take, for example, the nickname for traveler's diarrhea (sorry, the first thing that came to my mind). I've seen it written as Montezuma's Revenge (proper noun), as well as Montezuma's revenge (common noun). Which is correct?

What about a journalist referring to a specific set of letters sent by prisoners -- would he call them the Guantanamo Letters or the Guantanamo letters? Or is it simply a matter of context?

Answer

Proper nouns should name specific people, places or things - "specific" being the key word here. In some cases specificity is immediately apparent, e.g. in the term "Singapore Airlines" (after all, there is only one airline with that name!).

In other cases, there is a grey area and I would say that fame/notoriety also plays a role in determining specificity. To use your example, one would refer to the Guantanamo letters if they don't contain any content that is particularly newsworthy. If, on the other hand, these letters happen to disclose some salient information that could result in a political scandal, then these letters would quickly garner attention and become intimately linked to this political scandal in the minds of people. Such a link would imbue the letters with a specific quality that would warrant the use of a proper noun, i.e. the Guantanamo Letters.

Thus, some common nouns can become proper nouns over time as they gain specificity in the minds of people. This can be a very organic process that can leave plenty of room for interpretation as to whether something should be considered a proper noun or not.