Reference frames of a triaxially ellipsoidally symmetric mass distribution and its projected surface density. The transparent shaded gray ellipsoid represents a surface of constant density, with principal major ${\widehat{x}}_1$, median ${\widehat{x}}_2$, and minor ${\widehat{x}}_3$ axes marked red, green, and blue, respectively. The observer’s-frame axes are marked purple, with x₁′ and x₂′ in the plane of the sky and x₃′ oriented along the line of sight to the observer. The perimeter of the dark-gray ellipse in the background represents a corresponding contour of constant surface density, with plane-of-the-sky axes marked brown: x₁ along the major, and x₂ along the minor axis of the ellipse.