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Fischer projections: how to draw them and assign R/S

A Fischer projection is a flat cross that stands for a 3D carbon. It's the standard way to draw sugars and amino acids, and once you know two rules you can read R and S from it in seconds.

Stereochemistry · Updated 5 November 2026

What the cross means

The chiral carbon is where the lines cross. The horizontal bonds point towards you (wedges). The vertical bonds point away (hashes). By convention the carbon chain runs vertically, with the most oxidised carbon (such as CHO or COOH) at the top.

abcd=abcdhorizontal = towards you, vertical = away
A Fischer projection is shorthand for the wedge drawing on the right

Assigning R and S

  1. Rank the four groups 1 to 4 with the CIP rules (see R and S configuration).
  2. Find group 4 (usually H).
  3. If group 4 is vertical (pointing away), trace 1 → 2 → 3: clockwise is R, anticlockwise is S.
  4. If group 4 is horizontal (pointing towards you), trace 1 → 2 → 3 and reverse the answer: clockwise means S, anticlockwise means R.
CHOCH₂OHOHH
D-glyceraldehyde: OH (1) → CHO (2) → CH₂OH (3) is anticlockwise, but H is horizontal, so reverse it: (R)
COOHCH₃HH₂N
L-alanine: NH₂ (1) → COOH (2) → CH₃ (3) is clockwise, H is horizontal, so reverse it: (S)

The moves you can and can't make

MoveResult
Rotate the whole projection 180° in the planeSame molecule
Rotate it 90°The enantiomer (R becomes S)
Swap any two groupsThe enantiomer
Swap two pairs of groupsSame molecule
Hold one group still and rotate the other threeSame molecule

A handy trick follows from this: if group 4 is horizontal, swap it with the vertical group at the bottom. Now it's vertical, so read R or S directly, then reverse the answer because you made one swap.

D and L

For sugars and amino acids, look at the chiral carbon furthest from the top. If its OH (sugars) or NH₂ (amino acids) is on the right, it's D. On the left, it's L. Natural sugars are mostly D. Natural amino acids are L.

CHOCH₂OHOHH
D-glyceraldehyde: OH on the right
CHOCH₂OHHHO
L-glyceraldehyde: OH on the left

D/L and R/S are separate systems. They often match up for one family of compounds, but not always: L-alanine is S, while L-cysteine is R, because sulfur changes the priorities.

Quick answers

How do you find R or S from a Fischer projection?

Rank the groups 1–4. If the lowest priority group is on a vertical bond, 1→2→3 clockwise is R. If it's on a horizontal bond, the answer is reversed: clockwise means S.

What do the horizontal and vertical lines mean in a Fischer projection?

Horizontal lines are bonds pointing towards you, out of the page. Vertical lines are bonds pointing away from you, into the page.

Does rotating a Fischer projection change it?

Rotating it 180° in the plane gives the same molecule. Rotating it 90° gives its enantiomer.

Rotate and swap groups in Bondline's Fischer explorerFree, works offline, no account needed.

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