Disulfide refers to a functional group with the structure R−S−S−R′. The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. The connection is a persulfide, in analogy to its congener, peroxide (R−O−O−R′), but this terminology is rarely used, except in reference to hydrodisulfides (R−S−S−H compounds).