The structure of the Si(111)-root 21 x root 21-(Ag + Au) surface is studied using surface X-ray diffraction and fluorescence yield measurements. The fluorescence measurements indicate that there are three gold atoms in the unit cell. From the analysis of the diffraction data, it is shown that the gold atoms form a trimer with an interatomic distance of 2.8 angstrom and that the sides of the trimer are almost parallel to the Si(111)-1 x 1 fundamental vectors, similar to that in the Si(111)-root 3 x root 3-Au structure. (C) 2017 Elsevier B.V. All rights reserved.