Similar to some who have already chimed in ~*~ The pieces mentioned get added together as the temp drops.
40F+(4C) : OR gradient winter hat (bonnet), hooded OR echo
30F’s (0C) : Patagonia Hooded thermal weight (also LOVE the hood on this to retain the hat from sliding off and the tall neck), hooded Houdini
20F’s(-7C) : EE hooded torrid Apex
0-20F (-17C): MLD balaclava, extra hoody (nano air light) and Buff
Negative 20-0F (-26C) ski goggles
below 20 below : I sleep inside ;^)
honorable mention- the 40F EE Convert quilt I use is purposely X long, if need be (usually around 3am) I can pull the thing up and over my head and cinch down to a breathing hole. I was willing to add a few ounces for the full length zipper in order to fully encapsulate my sleep system at the colder end of the spectrum and glad I did.
In dead of summer 50F+ nights  I use a half quilt  with either the Patagonia hooded thermal weight or the EE Torrid for upper body/head. That season doesn’t last long where I live currently , maybe a few weeks.