Learn how to insert an invisible zipper on pillows trimmed with welting / cording.  Stitching a zipper so that it is virtually invisible in pillows makes the pillow look more professional. PROJECT TIP: Purchase an invisible zipper longer than what you actually need. The excess can then be cut off after being sewn to the pillow.
Video from  Haberman Fabrics.

If you would additional information about how zippers work and the parts of a zipper, visit: All About Zippers.