There are a number of other knots that do the same thing, but the butterfly knot tends to work better because it doesn't jam when strained and it's easy to untie.
Click here for a additional information and a complete treatment of this useful knot.
Step 1: Where Will It Be Tied?
Step 2: Form Loop 1
Step 3: Form Loop 2
Step 4: Working With Loop 2
Click here for Knot Tying Terminology.