The Shakespeare quadrangle is a region of Mercury running from 90 to 180° longitude and 20 to 70° latitude. It is also called Caduceata. The Borealis quadrangle is north of Shakespeare quadrangle. To the west is Raditladi quadrangle, and to the east is Victoria quadrangle. To the southwest is Tolstoj quadrangle, and to the southeast is Beethoven quadrangle. Mariner 10 imaging Prior to the images taken by MESSENGER, the only spacecraft images of Mercury were those taken by the Mariner 10 spacecraft, which made three passes of the planet in 1974–75 (Murray and others, 1974a,b; Strom and others, 1975a). Most images used in mapping the geology of the Shakespeare quadrangle were taken during the near-equatorial first pass, with close encounter or the dark side of the planet. The second, south-polar pass did not image the Shakespeare quadrangle at high resolution. High-resolution images of small areas within the quadrangle were also obtained during the third pass, when the spacecraft was on a near-encounter north-polar trajectory. Because the spacecraft viewed the same areas from different positions during the first and second passes, stereoscopic pictures are available for certain areas of the southern hemisphere; however, such pictures are not available for the Shakespeare quadrangle. All of the Mariner 10 passes occurred under similar lighting conditions. Across the Shakespeare quadrangle, these conditions varied from low light at the terminator near the west boundary to higher sun at the east boundary. Consequently, lighting conditions were favorable for determining fine-scale relief in the west, but progressively less so toward the east. Conversely, albedo features such as bright crater rays, which are conspicuous in the eastern part, become increasingly difficult to recognize westward toward the terminator. This range of lighting conditions across the quadrangle results in inconsistent geologic mapping, because topography, albedo, and surface texture are critical for characterizing individual materials units. The average resolution of the pictures used from the first pass is just over 1 km. Regional setting The dominant feature in the Shakespeare quadrangle is the Caloris Basin, 1,300 km in diameter. This impact basin is the largest and best preserved on the hemisphere of Mercury observed by Mariner 10. Almost the entire eastern half of the basin is in the Raditladi quadrangle; the west half was in the nightside hemisphere of Mercury during all the Mariner 10 passes, and part of the southern half lies in the adjacent Tolstoj quadrangle (Schaber and McCauley,