A stereochemical projection for an acyclic molecule (or portion of a molecule) where the main chain is represented by a zig-zag line in the plane and the substituents are shown above or below the plane.
Z06747.png
Source:
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2222 (https://doi.org/10.1351/pac199668122193)