Note: Do not rely on this information. It is very old.
MM, the 13th letter of the alphabet, was developed by the Phoenicians from the hieroglyphic symbol representing an owl. M is the labial nasal, related to b and p as n is to d and t. It frequently interchanges with n; cf tempt from Latin tentare with count from Latin comitem. It may also become b, as in marble from Low Latin marmorem. It has disappeared in some words, as in five (Gothic fimf.)