Tefillin are comprised of three main parts: the written Torah passages [parshiyot], the boxes [batim] that hold them, and the straps [retzu’ot]. The passages are inserted into the boxes, which are connected to the straps with which one fulfills the commandment: “You shall bind them” (Deuteronomy 6:6), i.e., tie the tefillin on the arm and on the head. The batim are hollow boxes made of the hide of a kosher animal. They are processed in accordance with precise instructions. After the passages have been placed inside the boxes, they are sewn with string made of animal sinews, and then the leather straps are inserted through them, by means of which the tefillin are bound to the head and the arm.

The boxes of the tefillin must be square. They should be made from the skin of a kosher animal and be dyed black.

The tefillin of the head and the tefillin of the arm are somewhat similar in external appearance, but in fact there is a major difference between them. Whereas the box for the head consists of four distinct compartments, with one Torah passage placed in each compartment, the box for the arm tefillin contains only one space, into which a single scroll of parchment, containing all four passages in sequence, is inserted.

The passages of the tefillin must be written by a scribe, with a quill and ink, on parchment. There are many halakhic requirements regarding the writing of the Torah passages. Accordingly, one must trust that the scribe is an honest and God-fearing man. Sometimes a scribe can make a mistake that disqualifies his work, and he alone knows that this has occurred and must make the painful decision to shelve the written passages over which he has toiled. After the writing, the passages are examined through a meticulous proofreading, in order to ensure that every letter was written correctly. Nowadays it is also common for a proofreading to be performed by means of computerized scanning, which ensures that there are no missing or additional words in the passages.

The first passage of the four is: “Sanctify to Me every firstborn” (Exodus 13:1–10), toward the end of which it is stated: “It shall be a sign for you on your arm and a remembrance between your eyes” (Exodus 13:9). The second passage is: “It shall be when the Lord will take you” (Exodus 13:11–16), which includes the command: “It shall be a sign upon your arm and an ornament between your eyes” (Exodus 13:16). The third selection is the Shema (Deuteronomy 6:4–9), which states: “You shall bind them as a sign on your arm, and they shall be for ornaments between your eyes” (Deuteronomy 6:8). Finally, the fourth passage is: “It shall be if you will heed” (Deuteronomy 11:13–21), which, near its conclusion, instructs: “And you shall bind them as a sign upon your arm, and they shall be as ornaments between your eyes” (Deuteronomy 11:18).

These four passages must be written in a specific order; otherwise the tefillin are invalid. Over the generations, disagreements arose over what the correct order should be. The leading opinions on this issue are those of Rabbi Shlomo Yitzhaki, known as Rashi, and his grandson, Rabbi Yaakov, known as Rabbeinu Tam. The halakha follows the opinion of Rashi, and these are the tefillin donned by the Jewish people in all communities around the world every day. Nevertheless, many people, after removing the standard (Rashi) pair, are scrupulous to don a second pair of tefillin that are made in accordance with the opinion of Rabbeinu Tam.

The tefillin straps are made of leather and they must be black. The minimum width of the straps is 9 mm. As for the minimum requisite length of the straps of the head tefillin, they should be sufficient to encircle the head of the person who dons them, and then they should hang down on both sides and reach his lower abdomen. The strap of the arm should be at least long enough to wrap around the arm, stretch it to the middle finger, and then twist it three times around that finger.

According to halakha, on the straps of the head and the arm tefillin must be knots with specific shapes, the tradition for these having been passed down by oral tradition. The knot on the arm tefillin is in the form of the Hebrew letter yod. In the case of the knot of the head tefillin, there is a dispute between the halakhic authorities as to the correct form. The two prevailing opinions are to tie the knot in the form of the Hebrew letter dalet or the final letter mem.

The tefillin straps are sacred, and therefore one must treat them with respect and be careful not to demean them by dragging them on the floor and the like.

In order to protect tefillin from damage and improper treatment, it is customary to place them in special plastic boxes, which are then inserted into a sturdy container or cloth or velvet bags or something similar.