Top, arm tefillin from the Cairo Genizah. The box is conical.
Middle, a drawing of tefillin made in 1725 in Amsterdam. The arm box is conical here, as well.
Bottom, modern tefillin. Note the arm tefillin is rectangular, not conical.
Please click the images to enlarge.
I post these pictures to show that our understanding of tefillin has changed over the centuries.
Modern arm tefillin are square or slightly rectangular. The Shulkhan Arukh, written near the end of the 1500s, codifies this as law. Yet the tefillin pictured above, worn 150 years after the Shulkhan Arukh was published, seem to disregard this as does the find from the Cairo Genizah.
Why? Any thoughts on this?