פּירוש אויף דברים 6:8

וקשרתם AND THOU SHALT BIND THEM [ … UPON THY HAND] — These are the Tephillin that are placed on the arm,

וקשרתם לאות על ידך, “you are to bind them (these words) on your hand;” in a single wrapping and a single knot. On the other hand, the phylacteries to be worn on your head, will have the shape of טוטפות, “frontlets worn between your eyes on your forehead,” i.e. four separate sections, as explained by Rashi.

Bind them as a sign upon your arm literally.

לטטפות בין עיניך AND THEY SHALL BE FOR FRONTLETS BETWEEN THINE EYES — these are the Tephillin that are placed upon the head. It is in reference to the number of the Scriptural sections contained in them that they are termed טטפת, for טט denotes "two" in Katpi and פת in Afriki denotes "two" (Sanhedrin 4b; cf. Rashi on Exodus 13:16).