The possible merohedral or hemihedral twinning operators for different point groups are: True point group Twin operation hkl related to 3 2 along a,b h,-h-k,-l 2 along a*,b* h+k,-k,-l 2 along c -h,-k,l 4 2 along a,b,a*,b* h,-k,-l 6 2 along a,b,a*,b* h,-h-k,-l 321 2 along a*,b*,c -h,-k,l 312 2 along a,b,c -h,-k,l 23 4 along a,b,c k,-h,l References: Yeates, T.O. (1997) Methods in Enzymology 276, 344-358. Detecting and Overcoming Crystal Twinning. and information from the following on-line sites: CNS site http://cns.csb.yale.edu/v1.1/ CCP4 site http://www.ccp4.ac.uk/dist/html/detwin.html SHELX site http://shelx.uni-ac.gwdg.de/_{rherbst/twin.html}