“Do not round off the corners of your head!” (Vayikra 19:27)
It is a negative commandment that one must not round off the corners of the hair of one’s head, as it says “Do not round off the corners of your head!”, and Chazal explained that the point is that a Jew is forbidden to shave off the hairs of his head to make a line between his forehead and the back of his ears, like the idol worshippers and their priests do even today.
Amongst the roots of this commandment are that one should keep away from and forget from our eyes and deeds anything which has the remotest connection to idol-worship or any of its subsidiaries. Being that this is the root of the commandment Chazal felt that it was necessary for them to state explicitly that shaving the whole head is also included in this prohibition, so that one should not say that since the reason for the prohibition is so that on should not be like them, and they would never shave their heads completely, that it should therefore be permitted. Chazal therefore had to tell us that this is also included. It may well be that the reason for this is that the Torah forbade even things which are only very slightly similar to the actions of the idol-worshippers.
Amongst the laws of this commandment are that the barber commits the crime, whereas the person being shaved only commits the crime if he helped towards the action of shaving. One who shaves a minor in this way, is also liable to receive lashes. As regards the size of the corner of the head, Chazal did not specify a size. The Rambam writes that “we have heard from our elders that one may not leave less than forty hairs”.
This commandment applies in all places and at all times, to men and not to women. Nevertheless, it is forbidden for women to shave men, even if they are minors, in this way.