466. Tzitzis and Shaatnez

Tzitzis 3:5

If a garment is made of wool, the white strings of the tzitzis should also be made of wool. If a garment is made of linen, the white strings of the tzitzis should also be made of linen. If a garment is made of some other material, the white strings should be made from that material, e.g., silk strings for a silk garment, goats' wool strings for a goats' wool garment, etc. If one wants to make white strings of wool or linen for any other type of garment, he may, because wool and linen can fulfill the obligation to have tzitzis for garments of their own material or for those of other materials. The other materials, however, can only fulfill the obligation to have tzitzis for garments of their own material.

Tzitzis 3:6

What about making wool strings for a linen garment or linen strings for a wool garment? One might think that this should be permitted because it is permitted to use shaatnez for tzitzis as we see from the fact that techeiles is made from wool strings and may be attached to a linen garment. Nevertheless, practice is not to do this because it is possible to make the white strings from the same material as the garment. If there’s a conflict between performing a positive mitzvah and observing a negative mitzvah, then if it’s possible to fulfill both of them, one should. If only one is possible, then performing the positive mitzvah takes precedence. In the case of tzitzis garments and strings, it is possible to fulfill the rules of both tzitzis and shaatnez.