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The Fischer projection, devised by Hermann Emil Fischer in 1891, is a two-dimensional representation of a three-dimensional organic molecule by projection. Fischer projections were originally proposed for the depiction of carbohydrates and used by chemists, particularly in organic chemistry and biochemistry. The use of Fischer projections in non-carbohydrates is discouraged, as such drawings are ambiguous when confused with other types of drawing.
All bonds are depicted as horizontal or vertical lines. The carbon chain is depicted vertically, with carbon atoms represented by the center of crossing lines. The orientation of the carbon chain is so that the C1 carbon is at the top. In an aldose, the carbon of the aldehyde group is C1; in a ketose the carbon of the ketone group has the lowest possible number (usually C2). 
A Fischer projection is used to differentiate between L- and D- molecules. On a Fischer projection, the penultimate carbon of D sugars are depicted with hydrogen on the left and hydroxyl on the right. L sugars will be shown with the hydrogen on the right and the hydroxyl on the left.
In a Fischer projection, all horizontal bonds project toward the viewer, while vertical bonds project away from the viewer. Therefore, a Fischer projection cannot be rotated by 90° or 270° in the plane of the page or the screen, as the orientation of bonds relative to one another can change, converting a molecule to its enantiomer. However, any rotation of 180° doesn't change the molecule's representation. Swapping two pairs of groups attached to the central carbon atom still represents the same molecule as was represented by the original Fischer projection.
When creating a Fischer projection for a carbohydrate with more than three carbons, each down carbon that would project away from you as viewed from the top in the Zig-Zag model must be turned around and oriented as towards your view. However this does not alter the Fischer projections for any previous carbons.
According to IUPAC rules, all hydrogen atoms should preferably be drawn explicitly; in particular, the hydrogen atoms of the end group of carbohydrates should be present. 
Fischer projections are most commonly used in biochemistry and organic chemistry to represent monosaccharides, but can also be used for amino acids or for other organic molecules. Since Fischer projections depict the stereochemistry (three-dimensional structure) of a molecule, they are very useful for differentiating between enantiomers of chiral molecules.
Haworth projections are a related chemical notation used to represent sugars in ring form. The groups on the right hand side of a Fischer projection are equivalent to those below the plane of the ring in Haworth projections. Fischer projections should not be confused with Lewis structures, which do not contain any information about three dimensional geometry. Newman Projection is normally used to represent the stereochemistry of alkanes.
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