tie the knot comes from the grecian tradition of tying a girdle around the bride for the groom to undo after the wedding

Isn't there also that tradition where the bride and groom hold hands and the vicar/priest/whatever ties a wide (purple) ribbon around their joined hands and forearms?

Leads up to: "Those whom God has joined, let no man tear asunder"

I've definitely seen this happen, but can't remember where.