Jewish Halachic Guidelines for Slips

When the dress or skirt is translucent (see through), here are a few solutions to make it opaque:

1. Line the dress or skirt with a material that makes it opaque
2. Wear a suitable slip (full length for dress, and half length for skirt)

The length of the slip should be a couple of inches shorter than the garment, and not shorter, otherwise a sudden movement that lifts up the garment will expose the knees, either indirectly via sunlight through the outer garment, or directly.

The slip should not be exposed from the outer garment, therefore wrap skirts or skirts with slits are not kosher to wear.

Source: Modesty, An Adornment for Life