Chapter 11.7 Detecting twinning by merohedry

Crystallography of biological macromolecules

Second Online Edition (2012)

Part 11. Data processing

  1. T. O. Yeates,
  2. Y. Tsai

Published Online: 14 APR 2012

DOI: 10.1107/97809553602060000836

International Tables for Crystallography

International Tables for Crystallography

How to Cite

Yeates, T. O. and Tsai, Y. 2012. Detecting twinning by merohedry. International Tables for Crystallography. F:11:11.7:311–316.

Author Information

  1. Department of Chemistry and Biochemistry, University of California, Los Angeles, PO Box 951569, Los Angeles, CA 90095–1569, USA

Publication History

  1. Published Online: 14 APR 2012


The chapter defines various kinds of twinning and focuses on the detection and analysis of twinning by hemihedry. The perfect overlap of lattices causes distinct intensities to be added together, thereby affecting the intensity distribution as well as the apparent symmetry properties of the crystal, to a degree determined by the twin fraction. Equations are presented for identifying the presence of twinning and estimating the twin fraction from an analysis of diffraction intensities. Complicating factors, including anisotropy, pseudo-centring and local noncrystallographic symmetry, are discussed.


  • twinning;
  • twinning by hemihedry;
  • twin fraction;
  • merohedral twinning