In the Marshall Islands, navigation was, and remains, an essential skill on which the lives of the navigators and all who sailed with them depended. In the past, knowledge of the art of navigation was a closely guarded secret handed down within certain chiefly families. To assist in recalling and imparting aspects of navigational knowledge, navigators constructed diagrams representing different portions of the archipelago. Made from the sticklike midribs of coconut palm fronds, these objects were memory aids, created for personal use or to instruct novices, and the significance of each was known only to its maker. The charts were exclusively used on land, prior to a voyage. To carry one at sea would put a navigator's skill in question.
The charts indicate the positions of islands, but they primarily record features of the sea. Marshallese navigation was based largely on the detection and interpretation of the patterns of ocean swells. Much as a stone thrown into a pond produces ripples, islands alter the orientation of the waves that strike them, creating characteristic swell patterns that can be detected and used to guide a vessel to land. It is the presence and intersection of swells and other aquatic phenomena, such as currents, that are primarily marked on the charts.