As we have learned, there are supposed to be five double knots on each of the tzitzis. Between each of the knots, the tzitzis are wrapped a specific number of times. These wrappings are known as krichos. According to the Shulchon Aruch, the wrapping should be done according to the following pattern: seven wraps between the first two knots, nine between the second and third, eleven between the third and fourth, and thirteen between the fourth and fifth--for a total of forty. Forty corresponds to the numerical value of the name of Hashem (twenty-six) plus the word echad (thirteen) plus one for Hashem, who is One. The Arizal teaches that the number of wrappings should be, consecutively, seven, eight, eleven and thirteen--for a total of thirty-nine. A third view maintains that there should be a total of twenty-six krichos, following the numerical values of the letters of the name of Hashem: ten, five, six and five.
[שו"ע יא, יד, ומשנ"ב ע; ביאורים ומוספים דרשו, 82]