Comedy as a dramatic structure does not simply refer to something that makes us laugh, although that can be a part of comedy. Instead, comedy as a dramatic structure can be defined as a movement from tension, usually family tension, to a complication of the tension "to the point of catastrophe," and then to finally a resolution ("Comedy: An Introduction"). Since the comic plots, especially Shakespeare's comic plots, surround the topic of family, the tension is always resolved through marriage; plus the state of marriage and even the concept of family are celebrated ("Comedy"). Shakespeare's comedies also contain certain plot devices, such as struggles that lovers must overcome; "separation and unification"; "mistaken identities"; "a clever servant," such as a fool; and "multiple, intertwining plots" ("Shakespeare plays--comedies, tragedies, histories").
Shakespeare's Twelfth Night certainly does fit all of the above definitions perfectly. For starters, we see the tension surrounding familial situations with respect to both Olivia and Viola. Olivia creates tension in the play through her prolonged mourning of her brother; her grief over her brother's death almost a year ago is so severe that she has completely ostracized herself from society, which is one reason why she is refusing the courtship of Duke Orsino. Viola creates tension when her response to being cut off from her family through being shipwrecked, orphaned, and separated from her brother is to disguise herself as a male servant in order to protect herself as an alone, wealthy, noble woman. We see her explain her motives for wanting to disguise her vulnerable, wealthy identity in her lines:
O that I served [Olivia]
And might not be delivered to the world,
Till I had made mine own occasion mellow,
What my estate is! (I.ii.43-46)
Viola's decision to disguise herself as a boy also leads to the mistaken identity plot so prevalent in Shakespearean comedy. Not only does Viola successfully disguise herself as a male servant to the point of Olivia falling in love with her, her twin brother is later mistaken for being her when he later arrives in Illyria after also having survived the shipwreck. Plus, his arrival in Illyria also leads to the brother and sister being united after having been separated during the shipwreck. Beyond the devices of struggling lovers, characters being separated and reunited, and mistaken identities, Twelfth Night also has multiple plots that intertwine and even Feste, the servant who is far more clever than the other characters, proving that Twelfth Night is indeed a comedy.