This is the second book of the Mishneh Torah of the Rambam, Rabbi Moshe ben Maimon. Laws of the Sefer Torah, Tefillin, and Mezuzah.
This chapter explains the proper placement of Tefillin and whether or not they are mutually exclusive. It covers how to recite the blessing and who speaks between each pair of Tefillin. All blessings are recited before fulfilling the mitzvah. It addresses who removes the Tefillin for storage, who designates a case for them, and the proper time for wearing Tefillin. It specifies whether nighttime is a Tefillin time and the law for someone who was in the synagogue and observed Shabbat or Yom Tov while still wearing Tefillin. It covers who is obligated to wear Tefillin and whether they must touch them constantly. It addresses whether a clean body is required and if one may sleep in them. If someone is wearing Tefillin and needs to use the restroom, what should be done? It covers who enters the restroom wearing Tefillin or who has intimacy with their wife while wearing Tefillin. It also covers the law regarding bathhouses and who may walk through a cemetery wearing Tefillin. A house where there is a Torah scroll or Tefillin allows for intimacy with one's wife. The greatness of the sanctity of Tefillin and the question of who recites the Shema without Tefillin.