Symbolism, Imagery, Allegory
When Bassanio chooses the correct casket and wins Portia as his wife, Portia slips a ring on his finger and makes a big speech about how she and everything she owns is now his property:
Myself and what is mine to you and yours
Is now converted: but now I was the lord
Of this fair mansion, master of my servants,
Queen o'er myself: and even now, but now,
This house, these servants and this same myself
Are yours, my lord: I give them with this ring;
Which when you part from, lose, or give away,
Let it presage the ruin of your love
And be my vantage to exclaim on you. (3.2.170-178)
Okay, fine. The ring is a symbol of Portia's love and commitment, and if Bassanio doesn't keep it on his finger, he's in big trouble. Bassanio says he's fine with this and promises that "when this ring / Parts from this finger, then parts life from hence" (3.2.187-188). No surprise here—couples have been exchanging rings for centuries.
Yet this ring also seems to symbolize much more. In "Portia's Ring: Unruly Women and Structures of Exchange in The Merchant of Venice," literary critic Karen Newman notes that, at first, the ring seems to represent Portia's submission to her soon-to-be husband and her willingness to become subservient when she marries (25). Note: Legally, in the 16th century, marriage made a man master over his wife and her property, which Portia acknowledges in her big speech.
Yet when Portia gives Bassanio the ring (and all of her worldly possessions and property), she also puts herself in a position of power, because she's giving her soon-to-be husband more than he can possibly give her in return (Newman, 26).
After all these dramatic declarations, we know what's bound to happen to that ring, right? Especially in light of Bassanio's questionable sincerity. (After all, his sole reason for traveling to Belmont is to snag a rich wife who will help him get out of debt. Portia is most definitely his meal ticket.)
Sure enough, Bassanio parts with the ring when Portia (disguised as a Balthazar) tricks him into giving it up as a token of appreciation after "Balthazar" saves Antonio's life (4.1). Hmm. What does all of this say about the couple's relationship and Antonio's commitment to it?
Portia also makes Bassanio beholden to her when she confronts him with the ring he willingly gave to "Balthazar." This isn't the only time Portia gets a leg up in the play. When, for example, she saves Antonio's life, she arranges it so that her husband and his best friend owe her more than they could ever repay.