Radially symmetrical, _star dunes_ are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around the margins of the sand seas, particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 meters tall and may be the tallest dunes on Earth. [Illustration: Linear dunes advance on small playas east of Lake Eyre in the Simpson Desert of central Australia (photograph by C. Twidale).] [Illustration: Linear dunes in the Western Desert of Egypt (photograph by Carol Breed).] Oval or circular mounds that generally lack a slipface, _dome dunes_ are rare and occur at the far upwind margins of sand seas. U-shaped mounds of sand with convex noses trailed by elongated arms are _parabolic dunes_. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and they are well known in coastal deserts. Unlike crescentic dunes, their crests point upwind. The elongated arms of parabolic dunes follow rather than lead because they have been fixed by vegetation, while the bulk of the sand in the dune migrates forward. The longest known parabolic dune has a trailing arm 12 kilometers long. [Illustration: Small crescentic dunes occur on the crests of these complex dome dunes of Saudi Arabia’s Empty Quarter (photograph by Elwood Friesen).] [Illustration: Ripples and horns of this crescentic dune in Egypt indicate that the dune is moving right to left (photograph by John Olsen).] Occurring wherever winds periodically reverse direction, _reversing dunes_ are varieties of any of the above types. These dunes typically have major and minor slipfaces oriented in opposite directions. All these dune types may occur in three forms: _simple_, _compound_, and _complex_. Simple dunes are basic forms with a minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed, and complex dunes are combinations of two or more dune types. A crescentic dune with a star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed.