Sidemount originated with a need to replace the standard twin set with a configuration that was more suitable for cave or overhead diving situations. Although most caves are relatively shallow, the linear depth is quite extreme as a result of large horizontal distances that are covered . Single 12l tanks were insufficient in regards to gas allowances and also safety (redundancy) considerations. However, a standards twin set configuration wasn’t feasible as sure bulk hindered their ability to enter most caves or pass restrictions. A new configuration was required.
Mounting the tanks on either flank of the diver drastically decreases the profile of the diver, with both tanks tucked nicely under each arm. The ability to un-clip the rear of the tanks allows even further mobility of the tanks. With the tanks positioned in front of the diver it further decreases the horizontal profile. This makes the diver profile not much bigger then the diver themselves, enabling them to fit through restriction not allowed by twin sets. Some configurations even allow the diver to unclip the tank completely and push it through small holes to be retrieved on the other side. Many of the best caves and wrecks in the world require the use of the sidemount configuration.