Tying the knot... LITERALLY?

Does anyone know where the term "tying the knot" originates from, and any good wedding vows to say whilst you are "tying the knot" that signify the actual tying of the hands together?
Update: LOL @ Sarah.... it could mean a ball and chain on the bride too :)

Seriously though, where does it originate from and whats good wedding vows or a poem to say while you tie the knot?
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