The trick to making your fingers look slender is to make them look longer. Well-kept, healthy nails with a little bit of growth are an easy way to elongate your fingers, but the growing-out process may take time. In the meantime, nail polish color can visually lengthen your nails and thus your fingers, making your digits appear slimmer in the process.
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Load up on nail polish colors that are very similar to your natural skin tone. By diminishing the line between your nail and finger, you visually elongate the finger through an optical illusion. Look for colors with pink or beige undertones and don't be afraid to swipe the color on your actual skin to test out how close the dried look is to your skin color.
The best way to make your nail look as long as possible is to use a clear polish. Be sure that your nail is properly maintained -- push back your cuticles, remove any snags and file your nail in a slightly rounded shape. A rounded fingernail with clear polish will give you the longest nail look, which will in turn appear to slenderize your finger.
A French manicure gives the look of an all-natural color, but adds an even and seamless white line that continues throughout each of your fingers. This also can make your nails appear healthier and longer. Be sure to keep the white portion of the manicure to a reasonable width. Too thin or too thick will look unnatural and could end up making your nail seem shorter.
Just like fashion allows you to use silhouettes and patterns to play up your assets -- or camouflage the areas you don't like -- so patterns on your nails can serve to visually elongate and slenderize your fingers. Use stripes from the nail bed to the tip -- either one solid stripe down the middle or multiple throughout the nail, depending on how detailed you can get. For this, you can use any color combination, but stripes in a nude color combo can do double duty in your quest to create the look of slenderized fingers.