diff --git a/src/crystal.f90 b/src/crystal.f90
index 32040bce4..62679e309 100644
--- a/src/crystal.f90
+++ b/src/crystal.f90
@@ -512,20 +512,20 @@ function crystal_C66_twin(Ntwin,C66,lattice,CoverA)
 
   select case(lattice)
     case('cF')
-      coordinateSystem = buildCoordinateSystem(Ntwin,CF_NSLIPSYSTEM,CF_SYSTEMTWIN,&
+      coordinateSystem = buildCoordinateSystem(Ntwin,CF_NTWINSYSTEM,CF_SYSTEMTWIN,&
                                                lattice,0.0_pREAL)
     case('cI')
-      coordinateSystem = buildCoordinateSystem(Ntwin,CI_NSLIPSYSTEM,CI_SYSTEMTWIN,&
+      coordinateSystem = buildCoordinateSystem(Ntwin,CI_NTWINSYSTEM,CI_SYSTEMTWIN,&
                                                lattice,0.0_pREAL)
     case('hP')
-      coordinateSystem = buildCoordinateSystem(Ntwin,HP_NSLIPSYSTEM,HP_SYSTEMTWIN,&
+      coordinateSystem = buildCoordinateSystem(Ntwin,HP_NTWINSYSTEM,HP_SYSTEMTWIN,&
                                                lattice,cOverA)
     case default
       call IO_error(137,ext_msg='crystal_C66_twin: '//trim(lattice))
   end select
 
   do i = 1, sum(Ntwin)
-    call R%fromAxisAngle([coordinateSystem(1:3,2,i),PI],P=1)                                        ! ToDo: Why always 180 deg?
+    call R%fromAxisAngle([coordinateSystem(1:3,2,i),PI])                                            ! mirror on habit (twin shear) plane
     crystal_C66_twin(1:6,1:6,i) = R%rotStiffness(C66)
   end do