For me it's from the collarbone up to the chin, especially the front. I can't stand having t-shirts with collars that sit anywhere in this zone. I remember as a kid the collars on my t-shirts would be soaking wet because I would hold them in my mouth to keep from touching my neck. My solution is to wear low cut or V-neck tops. I sleep on my side on two pillows, this means most of my neck isn't touching the pillow very directly, just the upper part and on the side. Also don't like the 'double chin' touching my neck when I look down so I have been 'mewing' for decades to avoid it. I only learned it's called mewing recently, and no it doesn't tone your jaw (not what I was doing it for anyway)