Easton's 1897 Bible Dictionary:

Frontlets
   occurs only in Ex. 13:16; Deut. 6:8, and 11:18. The meaning of
   the injunction to the Israelites, with regard to the statues and
   precepts given them, that they should "bind them for a sign upon
   their hand, and have them as frontlets between their eyes," was
   that they should keep them distinctly in view and carefully
   attend to them. But soon after their return from Babylon they
   began to interpret this injunction literally, and had
   accordingly portions of the law written out and worn about their
   person. These they called tephillin, i.e., "prayers." The
   passages so written out on strips of parchment were these, Ex.
   12:2-10; 13:11-21; Deut. 6:4-9; 11:18-21. They were then "rolled
   up in a case of black calfskin, which was attached to a stiffer
   piece of leather, having a thong one finger broad and one cubit
   and a half long. Those worn on the forehead were written on four
   strips of parchment, and put into four little cells within a
   square case, which had on it the Hebrew letter called shin, the
   three points of which were regarded as an emblem of God." This
   case tied around the forehead in a particular way was called
   "the tephillah on the head." (See PHYLACTERY.)