Are they smushed together to make a single material or do my 95% cotton 5% lycra undies have 5 strands of lycra for every 95 strands of cotton?
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A lot of this information isn’t correct. I have degrees in textile science.
Generally speaking those percentages are calculated by weight, starting at the stage of yarn spinning. Elastomeric yarns like Lycra are continuous filament products, either mono- or multi-, and those are combined / integrated into the short-staple fiber spinning process. There is a process called core spinning, where the elastic is put into the middle of a yarn and covered by the cotton or polyester or whatever fiber touches the skin. That’s why you don’t see or feel those yarns in the garments. If you isolate one and untwist it, you’ll usually find a white stretchy filament in the middle.
Other items, like 50/50 (or whatever percentage) cotton / polyester blend, are mixed by weight prior to yarn creation while they are still fibers, if the product is made with short-staple yarn. That way you get a more homogenous blend for both properties and dye consistency. Dyes that work on cotton don’t work with polyester because chemically they’re older opposites.
Anyone saying they “assume they just alternate yarns” or anything similar is 99% likely to be dead wrong for garment products. Also, sweaters are not woven, they’re knit. This is because all different yarn and fiber types have pretty different mechanical properties, especially in terms of elongation. Mixing very different properties are varying intervals will give a very inconsistent product, in terms of stretch, shrinkage, puckering, etc in all the processes that come after fabric creation. It just doesn’t work like that for the majority of cases.
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