Learn English – Is “Monday” a proper noun or a common noun

Abstract nouns refer to abstract things, concrete nouns to concrete things. ‘Table’, for example, is concrete. It’s something we can touch and bump into. ‘Thoughtfulness’, on the other hand, is not something we can touch and bump into. It’s abstract.

Proper nouns describe things or people that are unique, such as ‘Africa’, ‘Madonna’ or ‘Piccadilly’. By contrast, there are several continents, singers and streets, so ‘continent’, ‘singer and ‘street’ are common nouns.

‘Thoroughfare’ is a concrete common noun. We may not be able to touch it and bump into it in the same way we can touch and bump into a table, but it is clearly something which exists in the world outside our own minds. It's a common noun because there are lots of them. It is also a countable noun, but perhaps we should save that for another time.

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.