I often trim my slipcovers with welt cord (piping). It adds a subtle design detail and professional finish. When the cord is covered in the same fabric as the slipcover it blends right in.
Welt cord also adds a lot of function to a slipcover. It gives structure to the furniture body, cushions and arms. It defines a skirt line or hem edge. And, it adds strength to seams.
The right type of welt cord will hold its shape and look smooth and straight throughout the life of a washable slipcover. I always use washable, flexible, non-shrinking cord. It has a poly tissue core that’s wrapped with a braided cover.
What I NEVER use is plastic clothesline. Here’s why: