Undergraduates estimated the center of mass for three equidistant black-filled dots varying in size. The triangular configuration appeared at different orientations. When one dot was larger than the other two, size ratio alone influenced the results and the arm of the response distribution pointed to the largest dot. When two dots were large and equally sized, the distribution pointed downward and toward the dot nearest 180 degrees. When all dots were the same size, responses aligned along the triangle's horizontal symmetry axis. A combination of three factors explains the results: size ratio, extrinsic vertical and horizontal spatial axes and the orientation of the triangle's intrinsic symmetry axes. In a second experiment we varied dot lightness while holding size ratio constant. There was no strong grouping effect of dots with shared lightness values at least at the sizes and distances we employed. This suggests that size is more important than lightness in determining a participant's subjective perception of mass.