Detailed laws on Tefillin Retzuos / Tefililn Straps
Length of the Retzuos/Tefillin strap of the Shel Rosh Retzuos / Tefillin Straps
It is preferable for the right side of the retzuah to reach the place of circumcision and the left side to reach the navel. The right side should be longer then the left.
The bracha on the shel rosh can only be made if each of the straps extend from the knot two tefachim (approximately 6 inches) . If the Tefillin Strap is shorter than two tefachim on either side one may wear them in case of necessity, but one should consult a Rav regarding making a Bracha.
Length of the Retzuah Shel YadThe Retzua Shel Yad should be long enough to tighten around the upper arm wrap seven times around the lower arm, and then wrap three times around the middle finger.
If the strap is not long enough, a Brachah can still be made if the strap is long enough to tighten around the upper arm, do one wrap down the arm and wrap three times around the middle finger.
Width of RetzuosThe preferable width is 11mm according to the Chazon Ish and 10mm according to Rav Chaim Naeh. The minimal width for making a Brachah is 9mm. If a Retzuah has been squeezed together at one spot making it less than the minimal width but it can be pulled back in to the minimal width it is acceptable. If however it has been stretched out and is narrower than the minimal width at any spot it must be replaced.
The end of the Retzuos may be cut on an angle even though they will be less than the minimal width.
The blackness of the Tefillin Straps all Authorities agree that it is preferred (Lechatchilah) that the entire upper surface of the Retzuos must be black. Any non black spot within the required minimum length and width makes the Retzuah posul until repainted.
If there is a non-black spot on the Retzuos outside the minimally required length and width some authorities are of the opinion that the Retzuah it is still kosher though not preferred.
It is preferred to blacken the sides of the Retzuos. Retzuos are considered black when they appear so from about a foot and a half away. The part of the Retzuah inside the knots do not have to be black, the part that rests within the Tefillin box should be black.
It is very common for the Retzuos to need to be touched up from time to time from normal usage. To do this you can purchase a special Tefillin marker or ink which was made for the sake of Tefillin (Lishmah) and from kosher ingredients. Before you start you must say “Lesheim Kedushas Tefillin” “For the shake of the holiness of the Tefillin”. if you forgot to say it, then as long as you were aware of the Halachah that the painting must be done Lishmah it is kosher Lechatchilah. If however, you were not aware then they are only kosher bedieved, however, if you can scrape it off and repaint it, it will retain its Lechatchilah status.