Piping adds a nice finishing touch to projects like cushions, pillows, or upholstery. The cord that is inside piping is referred to as cording, welting, piping cord or welt cord. It comes in many thicknesses. 1/8” is a common size. The core can be made of various materials like cotton, cellulose paper fiber, or foam. Cotton will be soft and flexible, the foam is stiffer, and the cellulose (used in this demonstration) is in between.
I’m making a throw pillow to show how to sew the piping. See How to Sew an Envelope Pillow with Piping for the full tutorial.