Self-reference: Every letter is a word
Large dictionaries list each letter as a single-letter word. Each such word is defined as a noun, denoting the letter with which it is spelled. This blurs the use-mention distinction: one can write
“Psychology” starts with a “p”.
mentioning the letter p, or one can write
“Psychology” starts with a p.
using the word p.
Some single-letter words have additional meanings, describing an object that is shaped like the letter (”the road came to a ‘T’”, “an L-shaped couch”) or used in idiomatic expressions (”suits her to a T”).
These nouns often form the plural irregularly, with an apostrophe:
“Egg” has two g’s.