The arm tefilla has one compartment and one parchment and the head tefilla has four.
Let's say someone has only one type available. He has two head tefillin or two arm tefillin. In such a situation, may one place the head tefiila on his arm or arm tefillah on his head? Or should he wear only one tefilla - whichever is the appropriate one for that body part?
(Assume that the strapping is done correctly.)