In its original form, Muaythai consisted of an arsenal of nine weapons – the head, fists, elbows, knees and feet – known collectively as na-wa arwud. However in modern Muaythai, both amateur and professional, headbutting an opponent is no longer allowed. To strike and bind the opponent for both offensive and defensive purposes, small amounts of stand-up grappling are used: the clinch. Formal Muaythai techniques are divided into two groups: Mae Mai or major techniques and Luk Mai or minor techniques. Muaythai is often a fighting art of attrition, where opponents exchange blows with one another. This is certainly the case with traditional stylists in Thailand, but is a less popular form of fighting in the contemporary world fighting circuit. With the success of Muaythai in mixed martial arts fighting, it has become the de facto martial art of choice for competitive stand-up fighters. As a result, it has evolved and incorporated much more powerful hand striking techniques used in western style boxing and the Thai style of exchanging blow for blow is no longer favorable. Note: when Muaythai fighters compete against fighters of other styles (and if the rules permit it), they almost invariably emphasize elbow (sok) and knee (kao) techniques to gain a distinct advantage in fighting. Almost all techniques in Muaythai use the entire body movement, rotating the hip with each kick, punch, and block. The rotation of the hips in Muaythai techniques, and intensive focus on “core muscles” (such as abdominal muscles and surrounding muscles) is very distinctive and is what sets Muaythai apart from other styles of martial arts.