Woven webbing with a plastic feel used in some modern upholstery.  It can be fixed with staples or tacks, but staples are more commonly used with this product.  This is a cheaper alternative to back elastic webbing.  

Please Note: Always use a tack with an improved/larger head – small heads can tear through the webbing when it is under tension. Normally 13 mm improved is sufficient or 16 mm improved on very large, heavy frames.  Make sure the tack completely driven home by being sure you cannot feel the edges of the tack when you run your finger over it.