5. Making a knot for the sake of a Mitzvah:
A knot which is not Biblically considered a permanent knot but rather only Rabbinically is allowed to be tied for the sake of a Mitzvah, such as to make a knot in order to measure one of the Torah required measurements. Even to tie a professional knot is allowed if it is impossible to measure it otherwise.
 Admur 317:4; Michaber 317:1; Mishneh Shabbos 157a as rules the Rambam and Tur; M”B 317:13; Shaar Hatziyon 317:6
Other opinions: Some Poskim rule that according to Rashi [on Mishneh ibid] and Tosafus it is forbidden to tie a Rabbinical knot even for the sake of a Mitzvah. [Beis Meir, brought in M”B 317:13]
 Back then measuring was done through a rope have knots made on the various parts of the rope which represent different measurements.
The reason: The reason specifically by the laws of Kosher the Sages were lenient to permit tying for the sake of a Miztvah is because the Rabbinical prohibition is not due to a decree that one may come to do an Issur, but rather being it is similar to the Biblical Melacha, and for this reason we are stringent. [See Likkutei Sichos 14 p. 15]
 Admur ibid; M”A 317:3; M”B 317:12