An arbitrary convention according to which (+)-glyceraldehyde, now known to be (R)-2,3-dihydroxypropanal, was named d-glyceraldehyde (with the @E02069@ l-glyceraldehyde and its @R05025@ dl-glyceraldehyde) and taken to have the @A00020@ represented by the @F02391@ formula shown in the diagram.
The atom numbered 1 according to normal nomenclature rules is conventionally placed at the top of the main chain, which is drawn vertically and other groups are drawn on either side of that main chain. The convention is still in use for α-amino acids and for @S06088@.
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2208 [Terms] [Paper]