Years ago I remember being quite affronted when someone felt my shoulders and said my Trapezius muscle was tense big time.
We get used to the tension in our body, and regard it as normal.
When I'm teaching people to do
forward neck stretches , I get them in position with head bent forward and hands clasped over the back (now top) of their head.
I then ask them to relax and drop their shoulders down, so as to use the weight of their arms to stretch the muscles at the back of their neck.
Quite often, they drop their shoulders a little and need several reminders, dropping them a little more each time. They are not aware their upper Trapezius hasn't fully relaxed.
The Trapezius is the big triangular muscle attached from the top of our neck...
all the way down our spine...
...nearly to our waist, and passing across to our shoulder blade and collar bone.
I think it is overrated as a source of pain, being blamed when other muscles under it are responsible. These are Levator scapulae and the Rhomboids .
It is easy enough to check the upper part of it, by pinching between thumb and index finger. See the cervicogenic headache page .
Shoulder rolls are one exercise for this muscle.
Having someone knead it from behind is good.