A ball is a formal dance. "Masked" or in masquerade means in disguise, in costume and wearing a mask.
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King Gustav III in 1792. From Wikipedia: A masked ball took place at the Royal Opera House in Stockholm at midnight on 16 March 1792. Gustav had arrived earlier that evening to enjoy a dinner in the...
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If you are talking about 'Ella Enchanted' the book written by Gail Carson Levine and published in 1997, the link below will give you full information.