The Archery Manufacturers Organization is known as AMO. The group developed a number of standards, including bow length. It is almost always AMO bow length that people refer to when they give a bow length in inches.  The AMO bow length is the string length plus 3 inches. The AMO has been renamed the Archery Trade Association. An AMO 66 inch bow takes an AMO 66 inch string (actual length 63 inches)