In is the sixteenth letter of the Mandaic alphabet and stands where Aramaic, Hebrew and Syriac have ayin. It joins on both sides.
In Mandaic it is read as the vowel e as well as carrying the old pharyngeal value, another instance of the weakening of the Semitic gutturals that runs through the language. That double duty makes it one of the letters where the difference between the classical description and the living reading is widest.
Its name is one of the few built on i rather than on a, which puts it in the small group of irregular names alongside ushenna, aksa, it, dushenna and kad. Those six are the letters doing unusual jobs in the alphabet: the vowels, the old gutturals and the two ligatures.
A separate and much later sign, ain, borrowed from the Arabic letter of that name, sits at the very end of the alphabet and should not be confused with this one. In is the inherited sixteenth letter with an unbroken place in the sequence; ain has no inherited place at all and was taken in to write Arabic words. The two are recorded in this atlas as distinct, with a relation between them saying so, because the names are close enough to be run together in a list.
Because in joins on both sides it takes a full set of four shapes and never forces a break, where ain, the late borrowing at the end of the range, joins only to the right. That difference in behaviour on the page is one more thing separating the two signs beyond the closeness of their names.