For the source text click/tap here: Yevamot 4
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The mitzvah of yibum (levirate marriage) is an example of the classic rule aseh dokheh lo ta’aseh – that performance of a positive commandment can push aside a negative commandment.
Our daf discusses other such cases and their sources. One example is that of tzitzit, which, according to the Biblical commandment that is rarely kept in our day-and-age, requires fringes that are colored tekhelet. While a typical beged (article of clothing) discussed in the Torah is made of either wool or linen, according to the Gemara, tekhelet is wool.
Thus, we find ourselves in a situation where the mitzvah will be fulfilled by attaching wool fringes to a linen garment, a mixture that is ordinarily forbidden by the Torah (see Vayikra 19:19 and Devarim 22:11).
We explore the mitzvah of Tzitzis and its ramifications including the recent debate as to wearing them hanging outside of the clothing.