Gosh, after reading everyone else's layering system, I must be extremely warm natured. My set up 20F here in Memphis, TN USA (which by the way is certainly the coldest here)is as follows:
Bottom:
>thermal polyprop pants with convertable hiking pants over them

Top:
>long sleeve Patagonia Capilene 1
>fleece jacket
>if it's very windy/wet: Rain Jacket

For anything warmer than that, I might only wear the fleece jacket or just a homemade insulated vest. I'll leave the thermal pants off if I possibly can. Either they have always been tight and I never noticed or I've gained weight and didn't notice. smile
_________________________
I always forget and make it more complicated than it needs to be...it's just walking.