Fischer projection
/ FISH-er /
Drawing a three-dimensional molecule on flat paper is awkward, especially for sugars with a chain of several stereocenters. Emil Fischer invented a tidy shorthand that flattens the molecule into a simple cross of horizontal and vertical lines, letting you read off the configuration at every center at a glance. It is called a Fischer projection.
In a Fischer projection, each stereocenter is drawn as a cross. The vital convention is the orientation: horizontal lines point toward you (wedging out of the page), and vertical lines point away from you (behind the page). The carbon chain runs vertically, conventionally with the most oxidized carbon (like an aldehyde or carboxyl group) at the top. So a single cross is a 3D tetrahedron viewed from a fixed angle, drawn flat. To compare two Fischer projections, remember the strict manipulation rules: rotating the whole projection 180 degrees in the plane keeps the same molecule, but a 90-degree rotation or a single swap of two groups gives the enantiomer.
Fischer projections are the traditional language of carbohydrate and amino-acid chemistry, and they underlie the D/L system used to classify sugars (D-glucose, L-alanine) by the configuration of the bottom-most stereocenter. They are wonderfully compact, but their rigid 'horizontal toward, vertical away' rule is exactly what makes them treacherous: handle them carelessly and you silently turn a molecule into its mirror image.
In the Fischer projection of D-glyceraldehyde, the CHO is at top, CH2OH at bottom (the vertical chain pointing back), with H on the left and OH on the right (the horizontal bonds pointing toward you). The OH being on the right at the bottom stereocenter is exactly what defines it as the D sugar.
Horizontal bonds toward you, vertical bonds away — a flattened tetrahedron.
Never lift a Fischer projection off the page and flip it over, and never rotate it by 90 degrees — both operations secretly convert the molecule into its enantiomer. Only an in-plane 180-degree rotation, or an even number of group swaps, preserves the same configuration.