any of various valleys or trenches on the surface of the Moon. The term was introduced by early telescopic observers—probably the German astronomer Johann Schröter about 1800—to denote such lunar features. The word rima (from Latin, “fissure”) is often used for the same kind of features.

Rilles measure about 1–5 km (0.6–3 miles) wide and as much as several hundred kilometres long. They are divided into two main types, straight rilles and sinuous rilles, which seem to have different origins. Those of the first variety are flat-floored and relatively straight; they are occasionally associated with crater chains and sometimes arranged in an echelon pattern. Some of these structures are thought to be graben (horst and graben)s, elongated blocks of crust that have collapsed between parallel faults. Other straight rilles, some of which have branches—for example, Rima Hyginus and the rilles on the floor of the great crater Alphonsus—appear to be tension cracks in regions where subsurface gases have driven eruptions of dark material resulting in rimless vent craters.

Sinuous rilles resemble winding river valleys on Earth. They are thought to be similar to flow channels created by lava flows on Earth, but the shape of these lunar valleys is more meandering, perhaps because ancient lunar lavas were much less viscous than those now known on Earth. In 1971 Apollo (Apollo program) 15 astronauts explored the sinuous Hadley Rille and found a V-shaped valley filled with rubble from walls that appeared to contain exposed rock layers laid down by successive lava flows. Their observations, however, did not clarify the feature's origin.