מ

Transliteration: mem
Old Hebrew: 𐤌
Meaning: The thirteenth letter of the Hebrew alphabet — one of the five letters with distinct medial and final forms

The thirteenth letter of the Hebrew alphabet. One of the five Hebrew letters that have different forms for beginnings/middles and endings of words.

Pope Clement VIII, Jerome's Prefaces, On Worship

I. THE HELMETED PROLOGUE. — Jerome lists Mem among the five doubled Hebrew letters
"Furthermore, five letters among the Hebrews are doubled: Caph, Mem, Nun, Pe, Sade; for the beginnings and middles of words are written differently through these letters than their endings."