Final answer:
A taper is the vertical alternation made to remove width in the nape area, which creates a smooth transition in hair length.
Step-by-step explanation:
The vertical alternation made to remove width in the nape area is called a taper. This is a haircutting technique often used in barbering to gradually decrease the length of the hair from the top to the nape of the neck, creating a smooth transition. When employing a taper technique, hairstylists or barbers cut the hair so it blends or tapers into the skin, thus reducing bulk and creating a more defined, polished look around the neckline.