Nothing is common about the Common Berthing Mechanism. It works in space. Its job is to physically join two ISS elements together with an airtight seal between them.
The two major structures that make up the Mechanism are rings, one of which is termed the "passive half," the other the "active half." The former has capture latch fittings, alignment guides and nuts. The latter, or active half, has capture latches, alignment guides, powered bolts and controller panel assemblies.
The mating process begins after the protective Meteoroid Debris Deployable Mechanisms are removed from the Adapter seals. These four petal-like devices are made of Kevlar, the stuff of bulletproof vests.
In a sequence of events more precisely choreographed than a tango, the two rings -- each mounted at the docking or berthing ports of the ISS modules being connected -- are first brought to within a few inches of each other.