The Sisit strings are comprised of two parts: The P'til, which is the loose strings and the G'dil, which is the upper part made of knots and coils. From Torah law, it is not mandatory to have the intricate sequence of knots and coils that we customarily wear. It is sufficient to merely tie the strings in a double knot at the edge of the Tallit, wrap three coils and then tie another double knot. However, the Hachamim instituted to make four sets of coils, with each set having a different number of coils. The Shulhan Aruch states that the first set should have seven coils; the next set should have 9, followed by a set of eleven and thirteen. According to this, the sum total of coils is 40. However, Rabbenu HaAri said that the second set should have only eight coils. This is how most of our Sisit are tied. According to this method, the sum total of coils is 39, which is the numerical equivalent of "Hashem Echad"-G-d is One. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) found an allusion to this sequence in a Pasuk in Kohelet.
The Mishna Berura (Rav Yisrael Meir Kagan, 1839-1933) discusses a case in which a person does not have enough time to tie his new Sisit strings to a Tallit before Shabbat. It is prohibited to make Sisit on Shabbat, because it is a Melacha of tying and because it is considered completing a vessel. The Mishna Berura rules that he should simply attach the strings, make three coils and then tie a double knot. That would be sufficient at the minimal level. After Shabbat, he would complete the proper sequence of coils and knots. The other Poskim add that he should first tie a double knot at the edge of the Tallit and then wrap the coils.
The Kaf HaHaim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) cites the custom of the Sepharadim to create layered ridges when wrapping the coils, forming what looks like a staircase. This has Kabbalistic significance. Some Ashkenazim have the custom to wrap the coils according to the name of Hashem-YHVH, ten coils and then five, followed by six and five.
SUMMARY:
The string of the Sisit is wrapped in four sets of coils, according to a sequence of 7.8,11,13. The Sepharadim have the custom to wrap the coils in a staircase-like ridge.
If one did not have time to tie Sisit to a Tallit before Shabbat, he should tie the minimum double knot and then three coils followed by a double knot.
