This article is offered in the hope that it will give the reader a very basic understanding of the Mitzvah of Tefillin. Although the laws of STA'M are complicated there are certain basics of Tefillin that everyone should know.
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Batim

The structures associated with Tefillin, are the two Batim (Housings). The one placed on the head is called the Shel-Rosh and the one placed on the arm is called the Shel-Yad.

--Shel-Yad The Bayis (singular of Batim) is formed into a cube by pressing the leather around a solid wooden mold. After the leather hardens, the mold is removed.

--Shel Rosh The bayis of the Shel Rosh is not a hollow cube, but four separate compartments. Instead of a solid mold, a block with slits is used. The leather is pressed into each slit to form partitions inside the bayis. When the block is removed, the hardened leather is in the shape of a square Bayis made up of four smaller Batim. Part of the shaping and molding process includes the letter shin being embossed on both sides of the Shel Rosh.

The Batim are sealed by sewing them with Gidin - sinews, of a kosher animal. Twelve holes are drilled around the sides of the batim through which the Gidin are threaded. The gidin must be sewn in the shape of a perfect square, as the requirement of Rebuah also applies to the stitching of the Batim.

Ibud Lishmah
Batim are made from the hide of a kosher animal. The hide must be processed for the specific purpose of making Tefillin. This process is called Ibud Lishmah.

Rebuah
The law requiring the Batim to be squared is called Rebuah. All corners must also be squared. If not, the Tefillin may not be usable until they are repaired. They should be evaluated by a competent Sofer or Halachic authority.

The Writing

Tefillin scrolls contain four paragraphs, each containing a portion of the Torah referencing the commandment of Tefillin. They are as follows:
1. Kadesh (Exodus 13:1-10), outlining our obligation to remember the Exodus.
2. VeHaya Ki YeViacha (Ibid. 13:11-16), speaking of our obligation to transmit this tradition to our children
3. Sh'ma (Deut. 6:4-9), affirming G-d's unity and our mutual bond of love.
4. Ve'Haya Im Sho'moa (Ibid. 11:13-21), declaring man's responsibility toward G-d.

The parchment
The preparation of the Tefillin scrolls begins with the parchment. Parchment is the hide of a kosher animal, that has been soaked, stretched and sanded Lishmah - with the specific intent of writing religious scrolls. If "Lishmah" is omitted, even the most beautiful and carefully written Tefillin scrolls is invalid.
The parchment should not be coated with a glaze. The glaze tends to peel when the Tefillin Parshiyos are rolled, causing the letters to crack and become invalid.
The parchment is then scored with lines - Sirtut. This is to guide the Sofer in writing.

How long does it take to write Tefillin?
It usually takes 15 to 20 hours to write a set of Parshiyos for Tefillin. Consider the disappointment of the Sofer who, upon completion of his work, discovers an error which invalidates his hours of work. An unscrupulous individual may make an improper correction which could never be detected, but the Tefillin nonetheless would still be Posul - invalid.

The Sofer
Tefillin must be handwritten by a Sofer who is knowledgeable in the Halochos of writing STA'M. He should be an expert technician as well, for the task of writing Parshiyos is a difficult and responsible one. Each letter must be properly formed, and must be complete, without disconnections or cracks. Lines and letters must all be properly spaced. Each letter must be completely surrounded by parchment -Hakofas Gvil. No two letters may touch.
One of the most important aspects of Tefillin is the Sofer. He must be a man of integrity. To insure proper Kashrus and reliability, one should purchase Tefillin from dealers and Sofrim who have the highest recommendation, and who are, themselves, knowledgeable in the field of STA'M.

Hiddur Mitzvah
Beautifying the mitzvah. Each Sofer has his own individual writing style. While all may be technically Kosher, beauty and consistency may vary considerably. The skill and beauty of the writing will vary from one Sofer to another, as seen in the product samples. Hiddur Mitzvah has financial implications as well. The more beautiful the Tefillin or Mezuzah, the greater the cost.

Law of K'sidron
Tefillin and Mezuzah must be written K'sidron - in consecutive order. If a letter was omitted, it cannot be inserted later. In a Sefer Torah such corrections are permissible. However, in Tefillin and Mezuzah it is Possul - invalid.

Retzuos - straps
The construction of the Tefillin is completed with the addition of the two Retzuos - straps.
One Retzuah is threaded through the
Shel Rosh, and a knot is tied in the shape of the Hebrew letter daled. This Retzuah must be made so that the length of the right side extends beneath the hip and the left side to below the elbow.
The other retzuah is threaded through the Shel Yad. This Retzuah includes a knot in the shape of the Hebrew letter yud. It must be long enough to be wound seven times around the arm and three times around the finger.
Should the Retzuos show signs of wear, they should be shown to a Sofer. The color of Tefillin - Batim and Retzuos - is always black.

How often do I need to inspect my Tefillin?
Tefillin should be checked at least twice during a seven year period. Even if a Parshah was written correctly and later a letter became incomplete or cracked, it becomes Possul.
Tefillin should be thoroughly checked by a competent Sofer. The parts of the Tefillin subject to wear and tear must be blackened, repaired or replaced as needed.