Figure 59 is a schematic diagram consisting of seven rectangles arranged in four columns. The rectangles are connected by lines to form a tree structure that is designed to be read from left to right. Each rectangle represents a category of shape. In the first column there is a single rectangle labelled 'shapes'. In the second column there are two rectangles labelled '2D continuous' and '3D'. In the third column there are two rectangles labelled 'sheet' and 'bulk'. In the fourth column there are two rectangles labelled 'solid' and 'hollow'. From the rectangle 'shapes' a line goes to the right and splits into two lines. One line goes to the rectangle '2D continuous' and the other line goes to the rectangle '3D'. No line leaves the rectangle '2D continuous'. From the rectangle '3D' a line goes to the right and splits into two lines. One line goes to the rectangle 'sheet' and the other line goes to the rectangle 'bulk'. No line leaves the rectangle 'sheet'. From the rectangle 'bulk' a line goes to the right and splits into two lines. One line goes to the rectangle 'solid' and the other line goes to the rectangle 'hollow'.