Models
Each stereocenter is drawn as an ideal tetrahedron, with every substituent shown as one sphere coloured by the atom attached to the stereocenter (C dark teal, N blue, O red, F light green, Cl green, Br brown, H white). Bond lengths are drawn evenly for clarity. R or S is worked out from the 3D arrangement: with priority 4 pointing away, 1 → 2 → 3 clockwise is R. Swapping two groups, flipping a center, or rotating about the C–C bond rebuilds the drawings from the same 3D model. The enantiomer panel is a true reflection of the 3D model. The overlay test holds one molecule fixed (faded) while you turn the other. Green rings mark groups lying within 0.4 Å of their partners, and “Find best fit” calculates the rotation that lines up corresponding atoms as closely as possible (Horn's quaternion method). For molecules with two identical stereocenters, C2 and C3 are allowed to trade places. In the overlay test the solid copy keeps its own C2–C3 rotation, because conformations interconvert freely while configurations do not. That keeps the wedge–dash, Fischer and Newman views in step with the 3D one.
Conventions used
Priorities follow the Cahn–Ingold–Prelog sequence rules (atomic number, explore outward at ties, duplicate atoms for multiple bonds). In two-stereocenter molecules, the other stereocenter is ranked as a substituent like any other group. Natural L-alanine is (S) and D-glyceraldehyde is (R). The sign of optical rotation, (+) or (−), cannot be predicted from R/S; it has to be measured. Sources: R. S. Cahn, C. K. Ingold & V. Prelog, Angew. Chem. Int. Ed. 1966, 5, 385; IUPAC Nomenclature of Organic Chemistry (2013 Recommendations), P-92.