Retzuos are the black leather straps that are drawn through the back passageway of the batim to bind them on the head and arm. They are knotted in a special manner, according to the custom of one's family or community. They are made from the hides of a bull or cow. The leather is tanned similar to the way leather for purses and shoes are tanned. The difference is that the procedure for tanning retzuos (and batim and parchment) is strictly regulated by halachah. The completed and painted hides are hand-cut into straps by a Torah observant Jew who expresses that he is doing so for the sake of the holiness of tefillin.

There are two types of leather: the top hide known as elyon and split hide known as shpalt. The hides are divided widthwise, the upper section being more smooth, softer and better quality. The lower half is more grainy, tougher and lower quality. Shpalt is generally not recommended as the graininess causes the paint to crack, revealing the leather underneath. Retzuos made from this lower quality hide are less expensive, but have a very short lifespan.

Halachah requires that the upper surface of the retzuos be completely black. Often certain spots become worn or frayed and need touching up with special paint that is available in a paint or marker form. If such paint is not available many authorities allow for a regular, pitch black marker or the like to be used. One must say "lishem kedushas tefillin"--"for the sake of the sanctification of tefillin" upon touching up his batim or retzuos. If one has not done so, a sofer or rabbi must be consulted.