This overlay compares the relative depths of Back (II) and Back (III) up to a difference of 10 centimeters. Areas of blue and purple record where Matisse added between 6 and 10 centimeters of plaster to the cast of Back (II); areas of yellow and orange indicate where he carved away between 3 and 6 centimeters of the surface. The main focus of his work was the reduction of depth and expansion across the surface. Gray areas indicate more that 6 cm of difference.