The breast can be imaged from a number of angles. The mediolateral oblique (MLO) view is probably the most important and most common view taken followed by the cranio-caudal view (CC). Both views are shown below. Additional views and positioning (e.g., exaggerated cranio-caudal view laterally, XCCL) may be obtained in order to clarify a suspected abnormality detected during screening or physical examination.

When viewing a mammogram it is important to know the exact orientation of the image. The breasts are best viewed as symmetric organs.  Comparison of the right breast to the left breast is done for evaluation of symmetry.  Perceptual psychologists have shown that the eye can more easily perceive asymmetric densities when patterns are scanned in a mirror-image fashion rather than side by side.  Therefore, the conventional method is to evaluate mammograms in a mirror-like fashion with both the MLO and CC views mounted back to back.  Some radiologists mount the right mammogram on the right and the left one on the left  (shown in diagram below).  Others prefer to view the mammograms as if they were facing the patient with the left breast on the observer's right and the right breast on the observer's left.  

There are no hard-fast rules, and positioning of mammograms on a viewbox is a matter of individual preference.  Nonetheless, one's approach should be systematic to avoid mistakes.