Chlorine and bromine reversibly intercalate into graphite. The anions then intercalate between those triple layers and restore neutrality. A number of metal halides intercalate into graphite. Carbon anodes are traditionally used because of lithium's ability to intercalate without unacceptable volumetric expansion. It is able to intercalate into DNA at the cytosine-cytosine sites. Extra metal atoms from first transition metal series can intercalate up to 1&colon;3 ratio. In this case the relatively small lithium atoms intercalate between the layers of carbon. As another consequence of their planar structure and electron-acceptor properties, NDIs intercalate into DNA. For example, the planar structure allows angelicin to intercalate between the DNA bases. Some polypyridyl complexes intercalate into DNA and show promise as drugs. Volcanic rocks intercalate these units between Kabul and Khost.