Blouse with a Slimming Effect
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Can I wear a light silky turtleneck under my V-neck sweater or blouse or do I need bare skin for the slimming effect?
However, you have to be careful of layering turtlenecks or other garments under the "vee" as anything that breaks-up the flow of the vertical focus will cancel out the elongating effect. For instance, a dark-colored turtleneck (even mock-turtleneck) under a pale v-necked blouse or partially open shirt cancels out the effect.
If your concern is one of modesty (showing cleavage or simply too much bare breastbone) you can perhaps look for the now-popular insets that are available as modesty panels or perhaps employ a tee-shirt or higher-necked shirt in a flesh tone or paler, neutral shade. Such would keep your modesty, but not inhibit the benefits of the deeper "vee" shape.
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See also:
How to wear front buttoned blouses
How to dress to look slim
How to look fashionable as a plus size woman