Defined oligonucleosome model systems have been used to investigate the molecular mechanisms through which the core histone tail domains modulate chromatin structure. In low salt conditions, the tail domains function at the nucleosome level to facilitate proper organization of nucleosomal DNA, i.e. wrapping of DNA around the histone octamer. Mg2+ ions can substitute for the tail domains to yield a trypsinized oligonucleosome structure that is indistinguishable from that of an intact nucleosomal array in low salt. However, Mg(2+)-dependent formation of highly folded oligonucleosome structures absolutely requires the histone tail domains, and is associated with rearrangement of the tails to a non-nucleosomal location. We conclude that the tail domains mediate oligonucleosome folding and nucleosomal DNA organization through fundamentally different molecular mechanisms.