The main application of the results of this paper is to prove the existence of real valuation rings of the quotient field K of an excellent domain A having prescribed centers, ranks, rational ranks and residue dimensions. The major part of the paper develops the machinery necessary for this task. Let α be a point in the real spectrum of the ring A. Much of the paper is devoted to showing that α can be uniformized by means of quadratic transforms. Assume now that A is local. Write Gα for the topological space of all generations of α in the quotient field K, write Uα for the subset of elements βGα which uniformize α, and write Bα for the set of βGα which blow up α. The two main theorems state that the interior of Uα is dense in Gα and that the set Bα is dense in Gα.