When two people get married, they "tie the knot." Example: "After dating for five years, we finally decided to tie the knot." When you tie "a" knot you wrap two pieces of rope around each other in a certain way so that they will not come apart. When you tie "the" knot you make a promise that you will stay together with one person whom you love. Example: "Did you hear about Dan and Jenny? They finally decided to tie the knot." Two people are like two ropes and they get married to tie "the knot" which keeps them together. Example: "So, when are you two going to tie the knot?"