The nuclear drip line is the boundary delimiting the zone in which atomic nuclei lose stability due to the transmutation of neutrons, causing an isotope of one element to mutate into an element with one more proton. Atomic nuclei except for protium (the most common isotope of hydrogen which consists of a proton and no neutrons) contain both protons and neutrons—the number of protons defines the identity of that element (i.e., carbon always has 6 protons), but the number of neutrons within that element may vary (carbon-12 and its isotope carbon-13, for example). The number of isotopes each element may have is visually represented by plotting boxes, each of which represents a unique nuclear species, on a graph with the number of neutrons increasing on the X axis and number of protons increasing along the Y axis. The resulting chart is commonly referred to as the table of nuclides, and is to nuclear physics what the periodic table of the elements is to chemistry.