Universal topological properties of two-dimensional trivalent cellular patterns are found from shell analysis of soap froth and computer-generated Voronoi diagrams. We introduce a cluster analysis based on the shell model and find the universal relation ln͑a͞m 2 ͒ A 1 B ln͑m 2 ͒, with the generalized Aboav parameter a and second moment of the number of cell edge distribution m 2 . For the second, third, and fourth shells of the cluster, A and B are the same for all samples. Furthermore, A is increasing with shell number while B is a universal number, 20.90. For the first shell, the slope B is the same for soap froths, but slightly different from Voronoi graphs. DOI: 10.1103/PhysRevLett.88.138302 PACS numbers: 82.70.Rr, 02.50. -r, 05.70.Ln Two-dimensional cellular structure constitutes a large class of patterns with important technological and scientific applications. Soap froth, polycrystalline grain mosaics, and biological tissues are natural examples of random, space-filling cellular networks [1]. Since cellular structures exist on scales ranging from microscopic to geological, much work has been devoted to the search for universal geometrical characteristics. Despite the enormous difference in length scales and different physical forces driving the evolution of the networks, there exist certain universal topological laws governing their similarity. These laws leave aside metrical properties (e.g., sizes of cells) and address the probability distribution P n of the number n of edges of a given cell, or correlations between the numbers of edges of adjacent cells. One of the best-obeyed empirical laws is the Aboav-Weaire law [2,3] which states that on average the sum [M͑n͒n] of the number of sides of the cells immediately adjacent to an n-sided cell is linear in n:with the second moment m 2 Pǹ 3 P n ͑n 2 6͒ 2 being commonly used as a measure of the disorder. The notation M͑n͒ denotes the average number of edges of the adjacent cells to a given n-sided cell. The Aboav parameter a is a measure of nearest neighbor correlation and for soap froth a is approximately 1.There have been several important works published after the recognition of Aboav-Weaire law. Godreche et al. [4] have used planar graph theory in the context of counting planar Feynman diagrams with a cubic interaction to give an analytic expression of the Aboav-Weaire law in addition to the calculation of P n . However, these exact results are based on a particular model and cannot explain the vast diversity of natural cellular patterns with different Aboav parameters and second moments. Delannay and Le Caer [5,6] have studied the stationary topological properties of 2D cellular structures generated by random fragmentation in computer simulations, with very different a and m 2 that falls on a universal curve that is valid for many systems, both natural and computer generated [5 -8]. There are also many papers published on the two-cell correlation using maximum-entropy argument [9], but none of these theoretical efforts succeed in explaining the uni...