תשובות הגאונים, ח”ב, עמ’ 330

“כך היה גודל לאחר קשר התחתון חוליה אחת של לבן ועליה של תכלת וחוליה של לבן ועליה חוליה של תכלת ועליה חוליה של לבן ועליה של תכלת ועליה של לבן וקושר קשר העליון וזה הוא לא יפחות משבע ואם רצה להוסיף עד י”ג מוסיף כסדר הזה.”

Teshuvot HaGeonim, Vol. 2, p. 330

“This is how he would make the gedil: after the lower knot, he would make one white chulya, above it one Tekhelet chulya, above that one white chulya, then one Tekhelet chulya, then one white chulya, then one Tekhelet chulya, and above that one white chulya. He would then tie the upper knot. This is what is meant by ‘one should not make fewer than seven.’ If he wishes to add, he may add up to thirteen, following this same order.”

Notes:

1. A “single knot” is made as follows: wrap one white string around the other seven strings, then pass the end of that string through the wrap and tighten it.

2. A white chulya is made by wrapping one white string around the other seven strings three times. The longer string, commonly called the shamash, is generally used for this wrapping, and the same string can then be used for the remaining white chulyot as well, although there is no halachic requirement to continue using the same string.

3. A Tekhelet chulya is made in the same manner as a white one, except that a Tekhelet string is used for the wrapping.