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Fig. 1. —Drawing shows how to mount step-oblique mammograms on viewbox and perform triangulation. Progressive step-oblique images from 90° mediolateral (ML) projection to 0° craniocaudal (CC) projection, performed in 15° mediolateral oblique (MLO) increments, are mounted on viewbox with projection markers at top. Nipples point in same direction and are aligned horizontally. Note that straight line can connect lesion on all sequential step-oblique images, which permits triangulation as follows. Using subset of full complement of step-oblique images (i.e., lesion not yet identified on either 90° or 0° projection, or both), draw imaginary line through lesion on available images and extend line to left, right, or both to indicate expected location of lesion on orthogonal projections. In this case, lesion is in upper inner left breast.