Figure 3.

The standard coordinate frame in imgCIF/CBF aligns the X axis with the principal goniometer axis and chooses the Z axis to point from the sample into the beam, i.e. the ‘Source’ vector. If the beam is not orthogonal to the X axis, the Z axis is the component orthogonal to the X axis of the ‘Source’ (or ‘−Beam’) vector. The Y axis is chosen to complete a right-handed axis system. The origin is in the sample. It is important to note that the direction of the principal goniometer axis is a design choice in creating or even in configuring a crystallographic beamline. Even if we were to restrict our choices of principal goniometer axes to be horizontal, it is possible and equally valid to have CBF coordinate frames in which the Y axis points down, as in this figure, or to have CBF coordinate frames in which the Y axis points up, depending on the direction of the X axis.