The eye radical on the left, an upright pictograph of an eye. It tells you the character names a part of the eye — here the lid. It groups 睑 with 眼, 睛, and 眶.
Carries the sound. The reading shifted from qiān to jiǎn, paralleling other characters built on this element such as 检 and 脸; it adds no meaning to the eyelid sense.
