According to the Torah tefillin are worn on the “hand” and “between the eyes” (see 13:9). The Gemara defines the tefillin of the “hand” as being placed on the kiboret – the bicep, the muscle between the elbow and the shoulder – and the tefillin “between the eyes” as being placed on the skull on the location of the fontanel, the soft area of a baby’s head.
Various derivations are offered explaining why halakha rules that neither “hand” nor “between the eyes” are understood literally by the Sages. When the Gemara suggests that perhaps “tefillin of the hand” should be actually placed on the hand, and that “between the eyes” should be understood literally, it is not only a theoretical discussion. According to the Mishna in Massekhet Megilla (24a) during the time of the Mishna there were Jewish sects that disagreed with the traditional interpretations of the Sages and actually performed the commandment of tefillin in a literal manner. Among these sects, apparently, were early Christians.
Through the ages there were, nonetheless, different interpretations of the Sages’ definition, as well. We know of groups that understood the term kiboret as meaning the fleshy part of the lower part of the arm, between the elbow and the wrist, and actually wore tefillin in that fashion. Rabbeinu Tam came out strongly against this understanding and proved that only the upper part of the arm could be considered the kiboret.
With regard to placing the tefillin of the head on the spot where a baby’s head is soft, when a baby is born the bones of the skull are separated. The area where these bones will eventually meet are open so that the brain is not covered with bone, but there is room for the skull to grow and develop. The Gemara’s reference to the area where the baby’s skull is soft refers to the soft area towards the front of the head, which ordinarily closes up by the time the baby is one-and-a-half years old.