Electron micrograph of fibrin. The 23-nm period along the fiber axis is half the length of a fibrinogen molecule.

[From John L. Woodhead et al., “The Ultrastructure of Fibrinogen Caracas II Molecules, Fibers, and Clots,” J. Biol. Chem. 271(9):4946–4953, 1996, Mar 1. © American Society for Biochemistry and Molecular Biology.]