The color Yellow was used by the Army Cavalry and Armored units for hat cords since 1858. Hat Cords are also sometimes referred to as braids, hobbles, acorns, bands, or straps. Available in your choice of Standard 29" length or Combat 31" length (great for combat knots). These are also now available in the shorter 27" and 28" sizes, which work well for the smaller Cav Hat sizes.
To make a combat knot on your Cav Hat cord, simply tie a half-hitch knot on each side of the center slide (sometimes called a "keep") in the front of the hat.