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The longest carbon chain is drawn vertically. At each carbon atom under consideration the C--C--C of the main vertical chain is bent so that the top and bottom atoms are below the plane of the paper, and with horizontal X--C--Y angle (where X and Y are substituents) bent up so that X and Y lie above the plane of the paper.
Note: If there is more than one asymmetric carbon atom in the molecule the model must be turned over when the second asymmetric carbon atom is considered. Therefore, the Fischer formula of such a molecule does not look like the molecule in three-dimensional space.
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