The square script in which Aleph is written today is not the script of the earliest Hebrew inscriptions. Jewish tradition holds that after the Babylonian exile the Aramaic chancery hand of the Persian empire, standard by the Hasmonean period, displaced the older palaeo-Hebrew script, which survived afterward mainly on coins; the letter names, including Aleph, are older Northwest Semitic words carried over from that earlier writing.
A scribe copying a Torah scroll, tefillin or mezuzah must reproduce every letter to an exacting standard, and Aleph carries one of the best-known exceptions to ordinary sizing: the opening word of Leviticus, vayikra, God called, is traditionally written with a small Aleph, distinguishing it from vayikar, God happened upon, the verb used of the pagan prophet Balaam.
Midrashic tradition, cited by commentators including Baal HaTurim, reads the small Aleph as a mark of Moses's own humility, as though he wished to minimise the honour of being directly called by God and nearly wrote the more casual verb before yielding, under instruction, to the correct one at reduced size. A single malformed Aleph, like any other letter, is enough to invalidate an entire scroll under Jewish law.