Sinistral and dextral, in some scientific fields, are the two types of

chirality Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

handedness In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to it being stronger, faster or more dextrous. The other hand, comparatively often the weaker, less dextrous or simply less subjec ...

") or

relative direction In geometry, a position or position vector, also known as location vector or radius vector, is a Euclidean vector that represents the position of a Point (geometry), point ''P'' in space#Classical mechanics, space in relation to an arbitrary refer ...

. The terms are derived from the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different terms (such as

dextro- Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circular ...

and laevo-rotary in chemistry, or

clockwise Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...

and anticlockwise in

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...

) or simply use left and right (as in

anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...

). Relative direction and chirality are distinct concepts. Relative direction is from the point of view of the observer; a completely symmetric object has a left side and a right side, from the observer's point of view, if the top and bottom and direction of observation are defined. Chirality, however, is observer-independent: no matter how one looks at a right-hand

screw thread A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called ...

, it remains different from a left-hand screw thread. Therefore, a symmetric object has sinistral and dextral directions arbitrarily defined by the position of the observer, while an asymmetric object that shows chirality may have sinistral and dextral directions defined by characteristics of the object, regardless of the position of the observer.

Biology

Gastropods

Because the coiled

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses ** Thin-shell structure Science Biology * Seashell, a hard o ...

s of gastropods are asymmetric, they possess a quality called

–the "handedness" of an asymmetric structure. Over 90% of gastropod species have shells in which the direction of the coil is dextral (right-handed). A small minority of species and genera have shells in which the coils are almost always sinistral (left-handed). A very few species show an even mixture of dextral and sinistral individuals (for example, '' Amphidromus perversus'').''Amphidromus perversus'' (Linnaeus, 1758)

Flatfish

The most obvious characteristic of

flatfish A flatfish is a member of the ray-finned demersal fish order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migratin ...

, other than their flatness, is their asymmetric morphology: both eyes are on the same side of the head in the adult fish. In some families of flatfish, the eyes are always on the right side of the body (dextral or right-eyed flatfish), and in others, they are always on the left (sinistral or left-eyed flatfish). Primitive

spiny turbot The spiny turbots are a family, Psettodidae, of relatively large, primitive flatfish found in the tropical waters of the east Atlantic and Indo-Pacific. The family contains just three species, all in the same genus, ''Psettodes''. The common nam ...

s include equal numbers of right- and left-sided individuals, and are generally more symmetric than other families.

Geology

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

, the terms sinistral and dextral refer to the horizontal component of movement of blocks on either side of a fault or the sense of movement within a

shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shea ...

. These are terms of relative direction, as the movement of the blocks is described relative to each other when viewed from above. Movement is sinistral (left-handed) if the block on the other side of the fault moves to the left, or if straddling the fault the left side moves toward the observer. Movement is dextral (right-handed) if the block on the other side of the fault moves to the right, or if straddling the fault the right side moves toward the observer.

References

External links