How to read a Fischer projection

The convention

A Fischer projection is a stick figure of a stereocenter (or a chain of stereocenters). Each cross has a carbon at the center; the two horizontal lines are bonds wedging out of the page; the two vertical lines are bonds dashing into the page. The molecule is always drawn with the chain "eclipsed" — every neighboring carbon is anti-periplanar, never staggered.

D vs L sugars

For a sugar like glucose, look at the bottom stereocenter (the one farthest from the most-oxidized end). If its OH is on the right, it's a D sugar; on the left, L. Most natural sugars are D. For amino acids, the bottom stereocenter is the α-carbon: NH₂ on the left = L (the natural form for proteins), on the right = D.

Reading R/S off a Fischer

Because horizontal lines come toward you, the standard "lowest priority in back" trick is inverted. Trick: if the lowest priority group is on a horizontal line, read the 1→2→3 rotation as drawn and then flip the assignment. If the lowest priority is on a vertical line, read it directly.

Manipulating Fischers

Rotating a Fischer projection 90° flips every stereocenter (you turn a row into a column). Rotating 180° in the plane leaves every stereocenter unchanged. Swapping any two groups at one stereocenter inverts that stereocenter. Two swaps gets you back to the original.