If it is a classical guitar, it is best for girls to wear long skirts, especially long skirts with jumpsuits. You can watch Yang Xuefei's performance. Most of the clothes are long skirts.

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If it is a folk guitar, you can choose a simpler dress, such as white or light T with jeans, which are all good in length.

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If you are standing, you can also wear a jumpsuit skirt or a T-shirt with a denim skirt.

If it is an electric guitar, it depends on the style. If you are still calm, you can wear clothes similar to folk guitars.

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Assuming that the style is more rock, it is a bit unconventional. Of course it's not too fancy. After all, it's not a special performance by a special singer.