Nylon and polyester—ubiquitous in outdoor apparel—are both made from petroleum-based polymers, but they have different pros and cons. Nylon is generally stronger than poly. A 420-denier (denier measures thickness and durability) nylon thread, for example, will be as tear- and abrasion-resistant as a heavier 600-denier poly thread, making nylon preferable for items that see a lot of rough use, like shells and pants. Meanwhile, poly is softer and stretchier—making it more prevalent in baselayers, midlayers, and sleeping bag liners—and it’s easier to engineer with wicking and UV treatments than nylon. Plus, polyester is often a greener choice because it can be made from recycled plastic and is more easily recycled at the end of its life.