The term "smell letters" is not a standard linguistic term. It seems like you might be thinking of a mnemonic device for remembering the vowels (A, E, I, O, U).
Here's a breakdown:
* Proper Nouns: These are specific names of people, places, things, or organizations. They always start with a capital letter, regardless of their position in a sentence.
* Examples: John, London, Eiffel Tower, Google
* Common Nouns: These are general names for things. They don't start with a capital letter unless they're the first word in a sentence.
* Examples: man, city, building, company
Let me know if you have more questions!