Mixed Cloth

What constitutes as mixed Cloth that should not be worn?

ב"ה Shalom, My understanding is that you are referring to the prohibition of Shatnez, as mentioned in the Torah (Vayikra 19:19 and elsewhere) . The Torah prohibits the mixture of wool and linen. Whenever a thread of wool and linen have been joined, whether in a thread, cloth or garment, or whether a thread of both has been spun together woven together or sewn toge(See Rambam Kila'im 10:2-3, Shulchan Aruch Yoreh De'ah 300:1) ther, the combination constitutes shatnez and may not be worn. Once the threads of wool and linen have been put together even the most minute quantity of wool and linen together constitutes shatnez. (Shulchan Aruch Yoreh De'ah 299:1). The presence of Shatnez in a garment cannot be determined by reading the garment's label alone, but it must be inspected by a professional Shatnez tester, who knows where the presence of Shatnez is common such as seams and collars and other areas of each garment whose contents are not specified in the label. All the best