Stereoisomerism Fischer projection formulas

Fischer projection formulas can help us identify meso forms Of the three stereoisomeric 2 3 butanediols notice that only in the meso stereoisomer does a dashed line through the center of the Fischer projection divide the molecule into two mirror image halves... 

Fischer projection formulas can be used to represent molecules with several stereoisomeric centers, and they are commonly used for carbohydrate molecules. For other types of structures, a common practice is to draw the molecule in an extended conformation with the main chain horizontal. In this arrangement, each tetrahedral carbon has two additional substituents, one facing out and one in. The orientation is specified with solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

The specification of configurations in diastereomeric species is quite simple, with each chiral center being designated R or S according to the sequence rules when the Cahn-Ingold-Prelog convention is used. An extension of the Fischer convention to systems with more than one asymmetric center that is based on carbohydrate structures and terminology is still used in relatively simple cases. This convention can be illustrated with the same stereoisomeric 2,3,4-trihydroxybutanals just discussed. The 2R,3R and 25,35 isomers are d- and L-erythrose, respectively. The 25,3/ and 2i ,35 isomers are d- and L-threose, respectively. The Fischer projection formulas are shown below ... 

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