Around 1890-1900, organic chemists were getting used to the idea that organic molecules are three-dimensional things, and that different arrangement of substituents in space gives different molecules. But they were not at all used to thinking in three dimensions (in fact, the wedge-dash system of "perspective" drawing did not come into general use for at least another 50 years!)

Emil Fischer devised the system of Fischer projections, which allows a 3-D molecule to be correctly represented by a plane figure. There are two conventions associated with Fischer projections, but only the first is essential:

  1. All vertical lines represent bonds going away from the viewer; all horizontal lines represent bonds coming toward the viewer.
  2. The main chain of carbon atoms is laid out vertically by convention.
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