The Yolk system of creating the taper to the back of a kilt was used on the very first of what today we call the modern style Kilts. This is the system used by Terry Vargas of Bear Kilts.

The pleats are folded and sewn to a yolk which was then sewn to the waistband. The yolk formed the Fell and created the taper from the hips to the waist.

What is a yolk? Well, look at the back of a pair of jeans. See that triangular shaped piece in the back under the waistband? That is the yolk.