A petticoat adds volume under skirts and can impact the silhouette of an outfit. Her wedding dress had a petticoat underneath to create a princess-like shape.
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Not Outerwear
Remember, a petticoat is not usually visible; it's meant to be worn underneath another garment. Though her dress was thin, the petticoat underneath preserved her modesty.
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Modern Usage
While still used, petticoats aren't as common in everyday fashion but may be seen in formalwear. For the ball, she chose a gown with a petticoat to add elegance.