When Sisit are tied to a garment, it is customary to wrap one of the strings around the others in between the five knots of the Sisit, and different customs exist concerning the number of wrappings that one should make. The Shulhan Aruch (Orah Haim 11:14) writes that one wraps the string seven times after the first knot, nine after the second, eleven after the third, and thirteen after the fourth. These wrappings combine for a total of forty, which alludes to the phrase, "Hashem Ehad" ("God is one"), which has a total numerical value of thirty-nine. When we add one to include the Name of God, we arrive at forty.
The Hida (Rabbi Haim Yosef David Azulai, 1724-1807) cites a different view, which claims that one should make only twenty-six wrappings, to corresponding with the divine Name of "Yod-Heh-Vav-Heh."
Rabbi Haim Vital (1543-1620) reported that his revered mentor, the Arizal (Rabbi Yishak Luria, 1534-1572), held a different view, namely, that one should make thirty-nine wrappings to correspond with the precise numerical value of the phrase "Hashem Ehad." Whereas the Shulhan Aruch held that the wrappings should be done in the sequence of 7-9-11-13, for a total of forty, the Arizal maintained that the proper sequence is 7-8-11-13, for a total of thirty-nine.
All these views are certainly acceptable, though one should preferably follow the Arizal’s system of 7-8-11-13. This applies both to the Tallit Katan worn under one’s shirt, as well as to the Tallit Gadol worn during prayer.
The Shulhan Aruch records a custom to tie knots at the edges of all the Sisit strings, in order to prevent the strings from unraveling. The Mishna Berura (commentary by Rabbi Yisrael Kagan, 1839-1933) disapproves of this practice, but the Shulhan Aruch clearly endorsed it, and Rabbi Haim Vital reported that the Arizal would tie these knots. Nowadays, the wool used for Sisit strings is generally high quality so the strings do not generally unravel. Nevertheless, if one notices that a string has begun to unravel he may certainly tie knots to prevent the strings from unraveling further. These extra knots are entirely permissible and do not constitute a violation of "Bal Tosif," the prohibition against adding onto the Misvot.
Summary: Several different customs exist concerning the wrappings made around the Sisit when tying them to the garment. It is preferable to follow the Arizal’s custom, of making the wrappings in the sequence of 7-8-11-13. It is permissible to tie knots at the ends of the Sisit strings to prevent them from unraveling.
