Various special linear structures connected with covariance matrices are reviewed and graphical methods for their representation introduced, involving in particular two different kinds of edge between the nodes representing the component variables. The distinction between decomposable and nondecomposable structures is emphasized. Empirical examples are described for the main possibilities with four component variables.
"Linear Dependencies Represented by Chain Graphs." Statist. Sci. 8 (3) 204 - 218, August, 1993. https://doi.org/10.1214/ss/1177010887