The lower and upper walls match mutually to form a horizontal slit, which preven the light from being reflected by the tape and prevent the misreading of optical detector outside the cassette.

该上、下壁互相配合而形成一水平准直狭缝，以确保光源发出的光不在磁带上方或下方反射而引起位于磁带盒外的光检测器的误读。