Sidemount is a scuba diving

Scuba diving

Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....

 configuration which has tanks

Diving cylinder

A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....

 mounted alongside the diver, below the shoulders and along the hips, instead of on the back of the diver. It is a popular configuration with advanced cave divers

Cave diving

Cave diving is a type of technical diving in which specialized equipment is used to enable the exploration of caves which are at least partially filled with water. In the United Kingdom it is an extension of the more common sport of caving, and in the United States an extension of the more common...

, as smaller sections of cave can be penetrated and tanks can be changed with greater ease. Sidemount is also growing in popularity with divers who have difficulty handling the heavy double tanks commonly used by cave divers. Shortcomings of this diving style include a greater difficulty in sharing air while diving with team members using backmounted tanks. The UTD Z-System solved this problem.

Sidemount diving began in England, where it was originally used in partially submerged caves. It was an easier method of transporting diving gear between totally submerged sections of the cave (sumps). Americans began using sidemount in the mid seventies for the same purpose. In the early eighties, Wes Skiles

Wesley C. Skiles

Wesley C. Skiles was an American cave diving pioneer, explorer, and underwater cinematographer. Skiles lived in High Springs, Florida.-Background:...

, and other Florida cave divers began to use sidemount to explore cave passages that were too low for conventional back mounted tanks. They quickly found that the British style of sidemount didn't work well in low passages, and slowly developed a "Florida style" form of sidemount. Today, the British refer to this "Florida style" diving as "American sidemount".

Most sidemount rigs used to be home-made as there were no commercial production of kits like there were for standard scuba rigs. One of the later "pioneers" of sidemount diving was Bill Rennaker, who started making kits available with sidemount accessories such as a custom butt plate.
Dive Rite released the Transpac which was able to be converted for sidemount diving, and then later followed it up with the Nomad.
Expansion of tec rec sidemount into recreational and open water courses now occurs in several countries globally. Note that sidemount configuration is not just a positional concept only - correctly matching technical (or even recreational) rigs together with sidemount harnesses is different from merely shifting tank postions using standard recreational gear for convenience, (which is not true sidemount). Sidemount may be split into basic sidemount (single tank - left or right positioned, but nominally left), and advanced sidemount (dual redundant tanks and regs, one left post and one right post). "Recreational" sidemount unmodified rigs should not be used for tec rec activities, particularly in overhead environs. Further, standard reg configurations, split-fins and other recreational equipment is unsuitable for profile manoeuvres, S-drills and other safety skills which necessarily require advanced sidemount configuration. Instruction in sidemount is not just for skilled enjoyment, but for diver safety. This is further explained in a number of international instructional sites.^^
Golem Gear sells a similar rig called the Armadillo.