Top: geometry reconstruction of the loop in 3D (yellow line) using difference images of SDO/AIA 193 (top left) and STEREO/EUVI-A 195 (top right) views. The northern loop footpoint is in green, whilst the southern one is in red, in blue the loop centre. The vertical (red) and horizontal (blue) slit is over-plotted. Bottom: projections of the loop in different orientations of the Heliocentric Earth EQuatorial (HEEQ) coordinate system. Distance are expressed in solar radii (R_⊙) units. The 3D data points are plotted as yellow points and fitted by the black line. The green, red and blue arrows mark respectively the normal plane, the minor and major radius of the loop. The red and blue lines at the loop apex locate the vertical and horizontal slits.