10:6 A chatzitzah is an interposition, i.e., a thing that comes between two other things. When one puts on his tefillin, there must be no chatzitzah in between the tefillin and his body. One’s hair is not normally a chatzitzah, though a big, puffy hairstyle might present such a problem. The law of a chatzitzah only applies to the boxes of the tefillin, not to the straps. Therefore, if one has a bandage on his forearm or on the side of his head where the straps lie, it does not present a problem (Mishnah Brurah 27:16).
10:7 A person must be careful that the knot of his arm tefillin remains adjacent to the box (even when they are in the bag) and that the Hebrew letter yud formed by the knot be facing his heart. When the arm tefillin are put on, the part of the box where the strap is inserted goes on top. If a left-handed person must borrow a right-handed person’s tefillin (or vice versa) and the position of the knot cannot be changed, he may put the arm tefillin on upside-down out of necessity. (By doing so, the yud and the knot will be facing his heart.)