They work with Electric cone winders. There are mages showing the notch and the purpose of the notch. One is of a close-up of putting the pigtail in the notch. The other image shows a cone of yarn without a notch and the result of what can happen when putting yarn on a cone without the notch.
Cardboard Yarn Cones, Big Size Old Stock Yarn Cones, Craft Cones
Cardboard Cones for Crafts, Winding Yarn, Lot of 4, Works with
Cardboard Cones for Craft Projects (Package of 12)
Cardboard and Plastic Cones – Cotton Clouds Inc.
Other Needlecraft Supplies, Needlecrafts & Yarn, Crafts - PicClick
200g 0.45 Lb 100% Wool Yarn Cones for Tufting Gun,rug Merino Yarns
Cardboard Craft Cones FOR SALE! - PicClick
Cardboard Yarn Cones, Big Size Old Stock Yarn Cones, Craft Cones
Stock up on these incredibly versatile cardboard cones! A great addition to your craft supplies these cones can be used to make decorations with
Do It Yourself Cardboard Cones - Craft Kits - 12 Pieces