Salt-like carbides are composed of highly electropositive elements such as the alkali metals, alkaline earth metals, lanthanides, actinides, and group 3 metals (scandium, yttrium, and lutetium). Aluminium from group 13 forms carbides, but gallium, indium, and thallium do not. These materials feature isolated carbon centers, often described as "C", in the methanides or methides; two-atom units, "", in the acetylides; and three-atom units, "", in the allylides. The  graphite intercalation compound KC, prepared from vapour of potassium and graphite, and the alkali metal derivatives of C are not usually classified as carbides.