[2] The arm-tefillah (or shel yad [literally "of the hand"]) is placed on the upper (non-dominant) arm, and the strap wrapped around the forelimb, hand and middle finger; while the head-tefillah (or shel rosh [literally "of the head"]) is placed between the eyes at the boundary of the forehead and hair.

Deuteronomy 11:18, for instance, does not designate explicitly what specifically to "bind upon your arm", and the definition of "totafot between your eyes" is not obvious.

At least as early as the 1st century CE, many Jews understood the verses literally and wore physical tefillin, as shown by archaeological finds at Qumran[5] and a reference in Matthew 23 of the Christian New Testament.

Jeffrey H. Tigay argued that the word טוֹטֶפֶת ṭoṭepheth originally meant "headband", as ornamental bands encircling the head were common among Levantine populations in the biblical period.

The scholarly consensus is that טוֹטֶפֶת ṭoṭepheth is derived from a reduplicated root ṭ-p-ṭ-p meaning "to encircle" (related to Arabic ṭāfa "go around, encirlce, encompass") with the feminine suffix -t.[6] Interestingly, Rabbi Akiva[7] argued the word טוֹטֶפֶת ṭoṭepheth to be a combination of two foreign words: Tot meant "two" in the Coptic[a] language and Fot meant "two" in the "Afriki" language,[8] hence, tot and fot means "two and two", corresponding to the four compartments of the head-tefillin.

Menahem ben Saruq explains that the word is derived from the Hebrew Ve'hateif and Tatifoo, both expressions meaning "speech", "for when one sees the tefillin it causes him to remember and speak about the Exodus from Egypt".

[17] The Sefer ha-Chinuch (14th century) adds that the purpose of tefillin is to help subjugate a person's worldly desires and encourage spiritual development.

[18] Joseph Caro (16th century) explains that tefillin are placed on the arm adjacent to the heart and on the head above the brain to demonstrate that these two major organs are willing to perform the service of God.

For instance, Yehudah B. Cohn argues that the tefillin should be perceived as an invented tradition aimed at counteracting the popularity of the Greek amulets with an "original" Jewish one.

[23] Joshua Trachtenberg considered every ornament worn on the body (whatever its declared function) as initially serving the purpose of an amulet.

For instance, Numbers Rabbah 12:3 presents tefillin as capable of defeating "a thousand demons" emerging on "the left side", rabbis Yohanan and Nahman used their sets to repel the demons inhabiting privies,[25] whereas Elisha the Winged, who was scrupulous in performing this mitzvah, was miraculously saved from the Roman persecution.

[26][27] Also, tefillin are believed to possess life-lengthening qualities,[28] and they are often listed in one breath among various items which are considered amuletic in nature.

[33] Nowadays the boxes should be fashioned from a single piece of animal hide and form a base with an upper compartment to contain the parchment scrolls.

The Talmud specifies that tefillin straps must be long enough to reach one's middle finger, and records the practice of Rav Aha bar Jacob to tie and then "matleit" (plait?

[42] The four biblical passages which refer to the tefillin, mentioned above, are written on scrolls and placed inside the leather boxes.

[45] Talmudic commentators debated the order in which scrolls should be written in the hand tefillin and inserted into the four compartments of the head-tefillin.

[48][49] Nowadays, the prevailing custom is to arrange the scrolls according to Rashi's view, but some pious Jews are also accustomed to briefly lay the tefillin of Rabbeinu Tam as well,[46] a custom of the Ari adopted by the Hasidim, many Sephardic communities, and individuals within the Ashkenazic community.

[53] The placement of the protrusion of a tuft of calf hairs (se'ar eigel) identifies as to which opinion the tefillin were written.

[54] The legal duty of laying tefillin rests solely upon Jewish males above the age of thirteen years, women are exempt from this obligation.

[12] Though no such prohibition existed in ancient times,[12] since at least the Rema in the 16th century,[55] the prevalent practice among religious Jews has strongly discouraged women from wearing tefillin.

[64] In 2018, a group of students from Hebrew College, a non-denominational rabbinical school in Boston, created a series of YouTube videos to help female and transgender Jews learn how to wrap tefillin.

Observant Jews make a tremendous effort to don Tefillin at the appropriate time every morning,[70] even in crowded airports.

[72] On Chol HaMoed (intermediate days) of Pesach and Sukkot, there is a great debate among the early halachic authorities as to whether tefillin should be worn or not.

Those who forbid it consider the "sign" of intermediate days as having the same status as the festival itself, making the ritual of tefillin redundant.

[12] The arm-tefillin is tightened with the thumb, the blessing is said, and the strap is immediately wrapped around the upper arm in the opposite direction it came from in order to keep the knot tight without having to hold it.

Many Ashkenazim and Italian Jews wear the knot to be tightened (not to be confused with the knot on the base which is permanently tied and always worn on the inside, facing the heart) on the inside and wrap inward, while most Nusach Sephard Ashkenazim and Sephardim wear it on the outside and wrap outward.

[79] The knot of the head-tefillin sits at the back of the head, upon the part of the occipital bone that protrudes just above the nape.