Peroxides are a group of compounds with the structure R−O−O−R, where R = any element. The O−O group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen peroxide (H2O2), colloquially known simply as "peroxide". It is marketed as solutions in water at various concentrations. Many organic peroxides are known as well. Aside from hydrogen peroxide, some other major classes of peroxides are: * Peroxy acids, the peroxy derivatives of many familiar acids, examples being peroxymonosulfuric acid and peracetic acid, and their salts, one example of which is potassium peroxydisulfate. * Main group peroxides, compounds with the linkage E−O−O−E (E = main group element). * Metal peroxides, examples being barium peroxide (BaO2), sodium peroxide (Na2O2) and zinc peroxide (ZnO2). * Organic peroxides, compounds with the linkage C−O−O−C or C−O−O−H. One example is ''tert''-butylhydroperoxide.