Thirteenth letter of the Hebrew alphabet; the meaning of the name is "water," the primitive shape of the letter resembling waves (see Alphabet). "Mem" has two forms: one for the beginning or middle of a word (מ), and one for the end (ם). Its numerical value (in post-Biblical writings) is 40. Being a labial, it interchanges in the Semitic group of languages with other labials, that is to say, with פ, ו, ב and sometimes also with the liquids ר, נ, ל. In composition it appears as a prefix, denoting place, time, instrument, or agent, and is employed to form all participles except the "ḳal" and "nif'al," and numerous substantives.