Making welt cord, or piping, is simply the act of covering plain cording with a decorative cover, usually fabric. The cord itself is available in different diameters, from the size of thin string to that of fat ropes. Double cords are also available.
Welt cord, often called piping, is one mark of good quality upholstery. Welt adds strength to seams, and gives any upholstered piece a finished, professional look.
Welt cord is also used as a design element. For example, by using a contrasting fabric for the welt, different from the fabric you are using for the body of the piece, you can pop a specific color from the main body fabric, or emphasize the lines of a piece.
For a good idea of how the finished piece will look, fold the welt fabric diagonally. Then placing the body fabric on top of it. Allow just the folded edge of the welt fabric to show beneath the body fabric.