It is noted that the factors of software that determine or influence maintainability can be organized into a hierarchical structure of measurable attributes. For each of these attributes the authors show a metric definition consistent with the published definitions of the software characteristic being measured. The result is a tree structure of maintainability metrics which can be used for purposes of evaluating the relative maintainability of the software system. The authors define metrics for measuring the maintainability of a target software system and discuss how those metrics can be combined into a single index of maintainability.