Couplet Summary

Lines 13-14

For I have sworn thee fair and thought thee bright,
Who art as black as hell, as dark as night.

• Whoa. Where the heck did this come from? This has got to be one of the most jarring final couplets we have ever read.
• Up until now, we thought our speaker was just talking aloud to himself or something. Now, we find out he's addressing someone specific and that he's pretty ticked off at this unnamed person.
• By the way, this sudden and dramatic shift in tone is what's called a "turn" or a "volta." Almost all of Shakespeare's sonnets have one, but they often happen in line 9. Go to "Form and Meter" for more on this.
• So why's this dude so mad? And why does this turn come so late?
• At the very least, our speaker feels like he's been totally duped because he thought this person was beautiful and moral and he went around saying as much. But it turns out that this person is ugly and corrupt, inside and out. 
• The words "fair" and "bright" refer to physical looks and morals. In 16th-century England, light skin was considered the standard of beauty and it was often associated with moral goodness and chastity. Here, the speaker is saying that his beloved is the exact opposite, which makes us think he's been lied to and cheated on by someone with a dark complexion. 
• Given all the talk about how his desire is like a "disease," it's also possible he's accusing this person of giving him a venereal disease. 
• F.Y.I., because the speaker is insulting the addressee's dark physical features and making accusations of moral corruption, a lot of literary critics assume he's talking to the same mistress who pops up in other sonnets. She's often called the "Dark Lady" because she's got dark hair and skin (as described in Sonnet 130) and shady morals. 
• We tend to agree that the speaker is addressing the "Dark Lady" here but, feel free to disagree, Shmoopers.
• Let's do a quick sound check before we finish up. Did you notice how this couplet has a sound that's all its own? 
• Scholar Helen Vendler points out how the final two lines have perfectly balanced syllables, unlike the rest of the sonnet: For I have thought thee fair (6 syllables), and thought thee bright (4 syllables) / Who art as black as hell (6 syllables), as dark as night (4 syllables) (source).
• Why does this matter? Well, one effect is that our speaker sounds pretty rational here—as if he's having a big "Ah-ha!" moment. Even though he just went on and on about how he's been raving like a madman, he seems to know exactly what he's saying here.
• Also, this is the only place in the sonnet where our speaker uses all single syllable words. Each word this dude spits out of his mouth is sharp and forceful. The effect is pretty harsh and gives us a sense of how bitter this guy has become.