A proper adjective is an adjective derived from a proper noun. They usually begin with capital letters—for example:
Iranian embassy
Spanish galleon
Napoleonic warfare
Germanic tribes
Australian dollar
A noun modified by a proper adjective should not be capitalized. For example, Iranian Embassy and Spanish Galleon are incorrect.
In general, it’s best to avoid using a place name as an adjective when the name contains more than one word. You can get away with phrases like New York minute or San Francisco fog, but, especially when the name has a comma, using it as an adjective makes the sentence difficult—for example:
Nirvana, the Seattle, Washington band that had kicked off grunge’s breakthrough into mainstream music, was scheduled to headline the festival . . .
Some writers put another comma after the state, creating clunky sentences like this:
Both candidates mentioned meeting the Toledo, Ohio, man on the campaign trail, and tied him into their economic plans. [Daily Orange]
One way to fix sentences like these is to cut out the state name—the Seattle band, the Toledo man. If the city shares a name with other cities in other states, consider putting the state in parentheses—Charleston (West Virginia) man, the Columbus (Ohio) band.
