One of the more popular diameters used by many natural horse trainers is 1/4 inch. Stiff Rope which holds it's shape and form.
Rope Construction: Polyester Cover/Parallel Polyester core
When the core of the rope consists of one or more un-braided fibers running parallel to a braided “cover” the rope can be called a braided cover parallel core rope or cord. Halter cord uses this construction. Fibers used in the core of this rope are premium polyester fibers. Parallel core ropes are not considered “splicable.” Previously called 8TF.
Rope Fibers: Polyester
Due to its strength, low stretch, excellent sunlight resistance, and excellent abrasion resistance properties, polyester is a preferred fiber for many natural horse trainers.
2 and 4 Knot Horse Rope Halters
General Purpose Rope