Some Characteristic Warning Signs for Twinning  

1.	Metric symmetry higher than Laue symmetry
2.	Rint for the higher symmetric Laue group only slightly higher than for the lower symmetric one
3.	Low value of <|E2-1|>
4.	Apparent trigonal or hexagonal space group
5.	Systematic absences not consistent with any known space group
6.	No structure solution
7.	Patterson function physically impossible (for heavy atom structures)
8.	An unusually long axis
9.	Problems with cell refinement
10.	Some reflections sharp, others split
11.	K = mean(Fo2)/mean(Fc2) is systematically high for reflections with low intensity
12.	For all of the most disagreeable reflections Fo>> Fc.