"This is one of Shakespeare’s so-called transvestite comedies, a category that also includes 'As You Like It' and 'The Merchant of Venice.' These plays feature female protagonists who, for one reason or another, have to disguise themselves as young men. It is important to remember that in Shakespeare’s day, all of the parts were played by men, so Viola would actually have been a male pretending to be a female pretending to be a male." -- "Twelfth Night," SparkNotes.com