A Fischer projection is a real convention, drawn here — the cross is the stereocentre: horizontal = wedges (toward you), vertical = dashes (away).
Below are the sugars and amino acids under discussion, so you can read R/S and D/L off the cross.
The workhorses of Fischer projections — glyceraldehyde (the reference sugar) and an amino acid. Drawn live from SMILES.
1. A Fischer Projection Is a Flat Shorthand for a 3-D Stereocenter
The cross replaces wedge-and-dash: the crossing point is the tetrahedral carbon (never written), and the four groups sit at the four ends.
2. Horizontal Bonds Point Toward You; Vertical Bonds Point Away
The two horizontal bonds are implied wedges (toward you); the two vertical bonds are implied dashes (away) — a flat bowtie.
3. The Carbon Chain Runs Vertically With the Most-Oxidized Carbon on Top
The chain is drawn vertically with the most-oxidized carbon on top — CHO for a sugar, COOH for an amino acid — so drawings stay comparable.
4. D or L Depends on Which Side the Reference OH (or NH₂) Sits
At the bottom stereocenter, OH (sugars) or NH₂ (amino acids) on the right = D, on the left = L — a drawing label, not the same as R/S.
5. To Get R/S, Check Whether the Lowest Priority Points Toward or Away From You
Rank by CIP, then find the lowest-priority group (usually H):
• On a vertical bond it points away — read 1→2→3 directly (clockwise = R).
• On a horizontal bond it points toward you — read 1→2→3, then flip the answer.
6. Rotate 180° to Keep Configuration, 90° to Invert It; Swapping Two Groups Also Inverts
A 180° rotation preserves every stereocenter; a 90° rotation inverts them all, as does swapping any two groups at one center.
7. Stacked Stereocenters Give Diastereomers, Meso Compounds, and Erythro/Threo
Stack one cross per stereocenter: same-side groups are erythro, opposite-side are threo, and an internal mirror plane (like tartaric acid) makes an achiral meso compound.
8. Summary
Horizontal = toward you · vertical = away · most-oxidized carbon on top · right OH/NH₂ = D · R/S direct if lowest priority is vertical, flip if horizontal · 180° keeps, 90°/one swap inverts · stack crosses, watch for meso.
Quiz yourself
Tap a question to reveal the answer — free, no login.
The two horizontal bonds point out of the page toward you (implied wedges); the two vertical bonds point behind the page away from you (implied dashes). The carbon at the crossing is implicit.
D. For sugars you read the bottom-most stereocenter: OH on the right = D, OH on the left = L. D-glyceraldehyde is the reference, and most natural sugars are D.
S. A horizontal bond points toward you, the reverse of the standard "lowest priority in back" assumption, so you flip the apparent answer: clockwise-as-drawn actually means S.
No — a 90° rotation swaps horizontal and vertical bonds, inverting every stereocenter into its enantiomer. Only a 180° in-plane rotation preserves the configuration.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.