It is a truth universally acknowledged, that nerds wear glasses. Usually ugly, unflattering glasses. There are several types of glasses that are signifiers of deeply-entrenched nerd-dom:

Chunky black plastic (tape on the bridge optional), sometimes chunky tortoiseshell plastic instead—as worn by the classic Hollywood Nerd, though these have lately come back into fashion again, style being a Cyclic Trope. Coke-bottle glasses so thick they're opaque—have their own page at Opaque Nerd Glasses