In a monoclinic space group an orthorhombic lattice metric can be simulated when one of the following conditions is fulfilled:

i) a = c [e.g. in Wittmann & Rudolph (2007) Acta Cryst. D63, 744-749]

ii) the beta angle is close to 90° [e.g. in Larsen et al. (2002) Acta Cryst. D58, 2055-2059 ]

iii) c cos beta is about -a/2 [e.g. in Declercq & Evrard, (2002) Acta Cryst. D57, 1829-1835]The a and b axes of the orthorhombic cell are

identical to the monoclinic a and c axes, respectively. The length of the orthorhombic b-axis can also be calculated by "c(monoclinic)

cos(beta-90°) = 1/2b(orthorhomic)".

- Eleanor Dodson ccp4@ysbl.york.ac.uk

via jiscmail.ac.uk to CCP4BB

You might like to look at this..

It tries to explain likely twinning possibilities in P21.

If you get C2222 and P21, then probably a~=c - then Beta can have any value.

C222 axes are then always possible with a* +c* , a*-c*, b* all having angles ~ 90

Without twinning you wont get 222 symmetry though. Pointless helps here.When the twinning domains are superimposable in three dimensions with twinning fraction alpha, defined as the fractional volume of domains in the second orientation;cannot be detected by examining diffraction patterns, but can be detected by examining the intensity statistics (use the "Twinning Server")

It arises when noncrystallographic symmetry (NCS) operators are close to true crystallographic symmetry.