Final answer:
In facial treatments, a pillow is used to support the client's knees, arms, or neck for comfort. It ensures proper positioning and helps prevent strain.
Step-by-step explanation:
The piece of facial treatment equipment that can be placed under a client's knees, arms, or neck to keep them comfortable during treatment is a pillow. A pillow provides the needed support and comfort, which helps to relax the client during various treatments. It's especially important to ensure that the client's neck is properly supported to prevent any strain or discomfort, and pillows can be adjusted to suit individual needs for optimal relaxation and efficiency of the treatment.