"Also by convention, the cdr of the last cons cell in a list is nil. We call such a nil-terminated structure a proper list. In Emacs Lisp, the symbol nil is both a symbol and a list with no elements. For convenience, the symbol nil is considered to have nil as its cdr (and also as its car)."

https://www.gnu.org/software/emacs/manual/html_node/elisp/Co...