Here's why:
* Common nouns are general names for people, places, things, or ideas.
* Proper nouns are specific names for people, places, things, or ideas.
"Tunic" is a general name for a type of garment. It doesn't refer to a specific tunic, but rather to the category of garments that share those characteristics.
For example:
* Common noun: Tunic
* Proper noun: The Tunic of Saint Stephen (a specific, historical tunic)