Wool clothing with linen spandex, or vice versa:[1]
It is permitted to insert a linen string/elastic into wool pants for use of tightening the pants [as is common in sweat pants and hoods[2]] so long as it is possible to remove the string without needing to cut the sewing.[3] It is even permitted to make a single knot with the string upon wearing it. One however may not make a double knot on the string.[4] [It goes without saying that it is forbidden to attach the belt in a way that it cannot be removed unless one cuts the sewed material. However, the custom is to be lenient in all the above[5] so long as the belt is not sewed onto the pants.]    

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[1] Rama 300/4

[2] Rama ibid “The string goes from one side to the other similar to the leather strap in the Tefillin that goes through the Mabarta.”

[3] The reason: As since one is able to remove the belt/elastic without needing to udo the sewing it is similar to wearing a linen belt on a wool garment. [Rama ibid] This implies that by underwear or by certain sweatpants which the spandex is within the cloth it would be a problem, as one needs to first tear the cloth in order to remove it. However Tzaruch Iyun as later on we say that to place wool in a linen bag is permitted, and how is this any different? Perhaps in truth its only a problem if its attached to the linen, or if tied on both sides.

[4] The reason: As since one is not able to remove the belt/elastic without undoing the knot [it is therefore considered attached]. [Rama ibid]

[5] Shach 300/8

The reason: The above ruling is only according to the stringent opinion mentioned by the Michaber in the previous law. Practically the world is accustomed to be lenient in the above like the first opinion mentioned in Michaber ibid. [Shach ibid]