R32 Space Group (Crystallography)

Why R32 Space Group is so named? I mean, if *R* stands for Rhombohedral and *3* for Threefold Axis, what 2 stands for?


I don't see any Twofold Axis in the cell:


<http://img.chem.ucl.ac.uk/sgp/large/155az1.htm>


**EDIT:**


What does it means 1/2, 2/3 and 1/6 on the arrows?


R32 have two cells.


This one: <http://img.chem.ucl.ac.uk/sgp/large/155bz1.htm>


And this other: <http://img.chem.ucl.ac.uk/sgp/large/155az1.htm>


Are they equivalent?


**EDIT:**


I have traced some lines to mark the screw-axes and the twofold-axis, but i can't see any point that follow the simmetry operator and the element of simmetry. Maybe i don't understand how it works.


Which is the height of the green line? And, which are the points of the screw-axis of the yellow line?![enter image description here](https://i.stack.imgur.com/mnPQo.png)




The 2-fold axes are parallel to the "paper plane ", i.e. perpendicular to the three-fold. They are depicted as arrows, the half-arrows indicate screw-axes. The numbers next to it show the "height" of the axis, i.e. where the plane parallel to the paper-plane cuts the unit cell: 1/2 would be in the middle, 1/3 would be a third from the bottom/top, etc. (obviously one only gives relative numbers here, as the absolute unit cell length varies depending on the real-life crystal you are looking at).


Also have a look here for the symbols: <http://img.chem.ucl.ac.uk/sgp/misc/guide.htm>


Otherwise, the two links you provided are equivalent, as you can see from the pictogram in the lower lefthand corner - the first link is in the "tri-/hexagonal", the second link in the "rhombohedral" setting. The symmetry for both is obviously the same (otherwise they would not be the same space group), it is just a different way of depicting the spacegroup by changing the point of origin. 


See here how they relate to each other: <https://www.uwgb.edu/dutchs/symmetry/rhombo-lattice.htm>


This one should also be quite enlightening - but I am not quite sure whether it might be behind a paywall: <http://it.iucr.org/Ab/ch7o1v0001/sgtable7o1o155/>
(Alternatively, have a look at the hardcopy of the International Tables of Crystallography, Volume A and look up spacegroup 155 - any decent crystallography lab or library should have it).


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