Originally Posted by astonh
Actually, this isn't a response to the initial question but rather a tag on question to Silver's answer. I have some nylon thread at home that was supposed to be for a sewing project that didn't work. ANYWAY... how do you know when you should add this reinforcement? Should you reinforce on ALL socks to prevent them from wearing out as quickly? I am knitting my first pair of socks out of some GORGEOUS yarn and I want them to last as long as possible.
You should add reinforcements when:
-the yarn is very thin, soft or delicate (angora, cashmere, any non-"sock yarn" laceweight)
-you plan to wear the socks with any regularity
-you want them to last a long time without darning
Basically, it's never a bad idea to add nylon to socks. It will only make them stronger and more durable. But it's not always necessary either. Most "sock yarns" already have a bit of nylon in them for durability. You also don't need to add nylon to thicker and/or tightly spun yarns.
Generally, I only add reinforcements if I'm knitting socks with a laceweight other than a "sock yarn". But then, I don't actually wear socks very often.