To keep hair additions hidden, avoid positioning them too close to the surface of the head or:

Positioning hair additions too close to the perimeter hairline can lead to visibility issues, as this area is the most noticeable when styling hair. Proper placement requires ensuring that the additions are discreetly tucked away, maintaining a seamless blend with the wearer’s natural hair.

When hair additions are placed too close to the hairline, they can become more apparent during movement or when the hair is styled back, undermining the natural appearance that is essential in cosmetology. The goal is always to create a result that looks authentic, and strategically placing hair additions slightly further from the hairline helps achieve that.

The other options don't capture the critical nature of the perimeter hairline in visibility. For instance, positioning additions to the center of the scalp may not necessarily be visible if blending techniques are applied correctly. Additionally, placing them below the natural hair level can still result in noticeable lines if not flawlessly executed. Keeping hair additions out of sight is also too vague, as it doesn't address the specific challenge of placement related to the hairline. Thus, ensuring they are positioned away from the perimeter hairline specifically minimizes visibility and enhances the overall aesthetic.