 coming straight out towards the observer, 
y in the plane of the screen or paper pointing to the right and z upwards.  

However, if orthographic viewing is ON and if a positive number (i.e. 1, 2 or 
3 for x, y or z) is entered to choose the axis of rotation, the rotation will 
be about the orthographic axes - that is, a set of axes which were originally 
parallel to the observer axes, but were rotated -18.4 degrees on z and 9.5 on 
y.  In a way, this type of rotation can be thought of as the structure 
rotating in its own coordinate system, while the observer is watching it from 
an off-center position. 

To avoid this type of rotation when orthographic viewing is on, and to rotate 
about the cartesian axes of the observer (or screen or plotter), use a 
NEGATIVE number to specify the axis.  

[E.2] ROTATE ON STRUCTURE VECTOR. 

This will rotate about a rational structure direction [uvw], regardless of the 
current display mode or orientation of the structure.  The indices or 
coefficients refer to a coordinate system fixed with respect to the structure 
(always rotating with it).  The indices must be integers, but this is not a 
bar in practice to rotation about irrational vectors, since the indices may 
take all values between plus and minus 32K. 

[E.3] REMOVE PREVIOUS ROTATIONS 

This will counteract all rotations made by suboptions [E.1] and [E.2], as well 
as any made on-screen (see section IV-2) and take you back to one of two 
settings.  You are asked if you want to go back to the "original" orientation; 
this means the orientation before any rotations, including the alignment of a 
slice, the initial rotations of section [B.3.7.3] and the orthographic 
rotation, have been made.  If you answer "N" to this query, you will be 
returned to the "first" orientation, i.e. the one which was in effect after 
the three types of rotation just listed have been made, if any of them were 
actually used. 

[E.4] TURN ORTHOGRAPHIC VIEWING ON/OFF 

This will switch on and off the rotations of -18.4 degrees on z and 9.5 
degrees on y, applied in orthographic display mode. Previous rotations done 
with suboptions [E.1] and [E.2] remain in force.  This in effect changes the 
viewpoint of the observer between the orthographic position, down the vector 
[621] referred to the observer cartesian axial system, and a straight-on 
position, down the x axis. 

[E.5] ALIGN FACE/VECTOR IN SCREEN/PAPER.  

This will rotate a specified face (hkl) to lie flat in the screen or paper 
(face-normal parallel to x observer), or a vector to be perpendicular to the 
screen or paper.  To align a face, it is also necessary to specify a vector 
lying in the face which will be vertical (z direction). Conversely, to align a 
vector, it is necessary to specify a face, containing the vector, whose normal 
will be vertical. If a vector [uvw] lies in a face (hkl), the indices satisfy 
the condition  hu + kv + lw = 0. If orthographic viewing is in effect, it is 
turned off when the face is aligned, t  90.00000 120.00000
X,Y,Z;                                                                      
-Y,X-Y,Z; Y-X,-X,Z; Y,X,Z; -X,Y-X,Z; X-Y,-Y,Z;                              
            P P31M       
    1    1    0    0    10.00000
    2   -1    0    0    10.00000
    3    0    1    0    10