Translation:Shulchan Aruch/Orach Chaim/26
26: Law of a Person Who Has Only One Tefillin.
1 If he only has one tefillin, he should don the one he has and bless upon it, for each one individually is its own mitzvah. And the same law applies if he has both of them, and there is a circumstance where he is only able to don one, he should wrap the one that he is able.
2 If he is able to wrap that of the head only, he should upon it recite the blessing "...on the mitzvah of tefillin" only. Gloss: And for us that have the custom to say two blessings each day, one should bless on that of the head by itself two blessings, and if he dons that of the hand only, he blesses "...to place" only [Tur].