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This might be the most obvious reason to wear a slip: to prevent show-through of underwear, or to obscure the silhouette when in bright light. In this way, slips help preserve modesty and provide a little well-calculated mystery to the female form. A garment backed by a slip gains a certain class simply by association
Slips provide a couple different kinds of protection, to both skin and fabric. Occasionally a garment will require a slip in order to hang correctly and skim over the female form in the most flattering way possible. It maintains this form whether the wearer is stationary or in movement, and maximizes the positive impact of any garment.
Not all materials that are impressive to the eye feel pleasant against the skin. Heavy wool suiting or stiff brocades can itch or chafe without an appropriate lining or in-between fabric layer. Metallic threads can rub most unpleasantly, as anyone who has worn ornate costumes or prom dresses can attest to. Slips in this case are protective, keeping the skin irritant-free and making an otherwise uncomfortable dress wearable