Proper nouns as adjectives

Sometimes Proper nouns can become adjectives. Look at the following :
London taxi
Paris metro
Have you come across such words? Sometimes you will come across such proper nouns and you must recognise this and not get confused. 
Common nouns are also used and adjectives, for example:
a glass bottle 
a grass skirt
a garden gnome.
Pay attention to these and do not get confused.