I've been hunting for something to keep me warm under my daily wear (jeans and khakis) for the last year.  Up until now, only pure silk was the right weight and they were way too fragile.  Most long johns are mid-heavy weight and are made for people walking around snow in freezing temperatures.  These are made for athletics, gut work perfectly for people living/working in chilly environments.

This product is tougher than silk, but almost as light.  With long johns under my clothes, I can go around town (San francisco) without a coat and not feel cold when the winter drizzle arrives.  They seem tough, too.  I only buy products that go through both the washer and the dryer.

I've since ordered two more of the exact same thing.  Great product; great price.