Abstract. We present a little-known preconditioning technique, called support-graph preconditioning, and use it to analyze two classes of preconditioners. The technique was first described in a talk by Pravin Vaidya, who did not formally publish his results. Vaidya used the technique to devise and analyze a class of novel preconditioners. The technique was later extended by Gremban and Miller, who used it in the development and analysis of yet another class of new preconditioners. This paper extends the technique further and uses it to analyze two classes of existing preconditioners: modified incomplete-Cholesky and multilevel diagonal scaling. The paper also contains a presentation of Vaidya's preconditioners, which was previously missing from the literature. [8]) and multilevel-diagonal-scaling (MDS) preconditioners (see [10], for example).The principle goal of this paper is to bring these techniques to the attention of a wider community of researchers. By doing so, we hope to encourage further work in this promising area. The primary original content of this paper, analyzing known preconditioners using the support-graph technique, serves several purposes. First, the analysis of MICC preconditioners establishes bounds that have never been proved before. Second, it shows that the technique is more widely applicable than previously appreciated. Third, we feel that the new proofs provide useful insights into these preconditioners; these insights can be used to improve the preconditioners and to guide heuristics for the construction of additional preconditioners.A secondary goal of this paper is to provide a complete presentation of the support-graph technique and of Vaidya's preconditioners. Vaidya's important contribution has never been published. Although most of the theory that he uses is presented in Gremban's PhD thesis [6], Vaidya's preconditioners have not been described in any published form. We seek to rectify this situation. Our complete presentation of the support-graph technique is necessary since some important portions of the