ABSTRACTOrigin of the longitudinal dune form is reviewed. The spectacular longitudinal dunes of the Simpson desert are caused by helicoidal wind flow of the prevailing south‐southeastern winds, acting upon ancient alluvial plains. Critical in this theory is the evidence from tuning‐fork junctures, which open upwind as do similar features in other geologic realms of primarily one‐directional currents undergoing second‐order helicoidal circulation.A summary of previous work on using grain size to distinguish beach from dune sands reveals the consistent good sorting and positive skewness of coastal dunes. Simpson desert dunes are also positively‐skewed, but are not as well sorted as coastal dunes because they are only recently derived from poorly‐sorted alluvial source sediments. Dune crests are coarsest and best sorted because they are made out of the most easily saltated fine sand (about 2.5 ø); windward flanks, leeward flanks, and reg are progressively finer and also show regular and explainable changes in the higher‐order grain‐size parameters. Both dune and reg sediments are made of polymodal mixtures, giving rise to a “quantum theory” o