Is it permissible to shave the neck with a razor?
It is permitted to shave the back of the neck with a razor, for the prohibition applies specifically to the beard, which is limited to the front of the face.
At the front of the neck, one may not shave the beard area, down to the area opposite the throat (adam’s apple). Beneath this one may use a razor.
There are opinions that are stringent, and rule that one may not use a razor in any area (see Rashba Vol. 4, no. 90; see also Beis Yosef, Yoreh De’ah 181, and Bach; see Hadras Panim Zakan, p. 502). The common custom is to be lenient, however, for the throat area it is preferable to avoid using a razor.