Types of Tefillin Straps / Tefillin Retzuos
In general there are two types of Tefillin Retzuos / Tefillin Straps available, both made from the hide of a cow or bull. The hide is split width-wise the straps made from the upper outer half of the hide is called Elyon they are softer, smoother and much longer lasting. The straps made from the lower inner half is called Shpalt, it is more grainy, cracks easily and does not last as long ( We do not sell these low quality Retzuos).
Tefillin Retzuos / Tefillin Straps are made either tanned and painted by hand or machine. Although there is generally no difference in the quality, it is preferable to use hand made, since there are Halchic Authorities that hold that machine made is only kosher bedieved (not Halchically preferred.)
The straps may not be narrower then nine millimeters for Ashkenazim a width of eleven is preferable. And for Sephardim ten is preferred. However the standard Retzuos are generally cut at approximately 15mm to allow for eventual stretching, Standard size batim range from 33-40mm If you have smaller then standard size Batim you may want to consider 13mm or if you have a larger then standard size Batim although not necessary you may want to consider 18mm.
It is a Halachah given to Moshe at Sinai that the Tefillin Retzuos / Tefillin Straps be black. In order assure that the Retzuos retain their blackness, We have Retzuos that are dyed through and through. These Retzuos are also known as black on black. The black on white standard straps have just a thin layer of paint on the top. Which eventually cracks or gets worn out and white strap underneath is exposed. The Black on Black are more expensive, however considering the fact that you will never enter a doubtful situation as to the blackness of your straps it may be something you will want to consider.
to purchase new Tefillin straps follow this link