Responsa for Kilayim 9:9
הַלְּבָדִים אֲסוּרִים, מִפְּנֵי שֶׁהֵם שׁוּעִים. פִּיו שֶׁל צֶמֶר בְּשֶׁל פִּשְׁתָּן אָסוּר, מִפְּנֵי שֶׁהֵם חוֹזְרִין כָּאָרִיג. רַבִּי יוֹסֵי אוֹמֵר, מְשִׁיחוֹת שֶׁל אַרְגָּמָן אֲסוּרוֹת, מִפְּנֵי שֶׁהוּא מוֹלֵל עַד שֶׁלֹּא קוֹשֵׁר. לֹא יִקְשֹׁר סֶרֶט שֶׁל צֶמֶר בְּשֶׁל פִּשְׁתָּן לַחְגֹּר בּוֹ אֶת מָתְנָיו, אַף עַל פִּי שֶׁהָרְצוּעָה בָאֶמְצַע:
Felted [wool beaten and presed, not spun or woven] materials are forbidden [if mixed with flax] because they are combed [together]. An edging [stitch that finishes off the woven cloth to prevent fraying] of wool on linen is prohibited, because it interlaces the weave [it resembles weaving]. Rabbi Yosi says, Purple woolen tie-cords are prohibited [to be tied around a linen robe], because they are stitched on before being tied [so as not to fall off the robe]. One may not tie a strip of wool [material] to [a strip] of linen [material] in order to gird one's loins, even though there is a strap [of leather] in the middle.