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Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction.
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The terms are derived from the Latin words for "left" (sinister) and "right" (dexter).
Other disciplines have different terms (such as dextro- and laevo-rotary, in chemistry, or clockwise and anticlockwise in physics) or simply use left and right (as in anatomy).
Relative direction and chirality are distinct concepts. Relative direction is from the point of view of the observer. A completely symmetrical object has a left 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, it remains different from a left-hand screw thread. Therefore, a symmetrical object has sinistral and dextral directions arbitrarily defined by the position of the observer, while an object that has chirality can have sinistral and dextral directions defined by characteristics of the object, no matter the position of the observer.
Geology
In geology 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. These are terms of relative direction, as the movement of the blocks are 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.
Gastropods
Because the coiled shells of gastropods are asymmetrical, they possess a quality called chirality, the "handedness" of an asymmetrical structure.
Over 90% of gastropod species have dextral (right-handed) shells in their coiling. A small minority of species and genera are almost always sinistral (left-handed). A very few species show an even mixture of dextral and sinistral individuals (for example, Amphidromus perversus).
Flatfish
The most obvious characteristic of flatfish, other than their flatness, is asymmetry because 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 turbots include equal numbers of right- and left-sided individuals, and are generally more symmetrical than other families.