D-value is the RA value for the root – the space at the bottom of a justified map – of a diamond-shaped pattern. This has nothing to do with geometric shape. It simply means a justified map in which there are k spaces at mean depth level, k/2 at one level above and below, k/4 at two levels above and below, and so on until there is one space at the shallowest (the root) and deepest points. (See: Diamond Graph)

Sources Hillier, B. & Hanson, J. (1984), The Social Logic of Space, Cambridge University Press: Cambridge. pp.111-112