In our INCOSE IS 2015 paper, “An Initial Ontology for System Qualities,” (SQs), we provided an IDEF5 class hierarchy of upper‐level SQs, where the top level reflected classes of stakeholder value propositions (Mission Effectiveness, Resource Utilization, Dependability, Flexibility), and the next level identified means‐ends enablers of the higher‐level SQs. In experimenting with, refining and formalizing the ontology, we focused on a depth‐first approach on a chosen SQ: Maintainability. It is key to reducing 75% of most systems’ life cycle costs. Also Maintainability plays key roles in three of the four top‐level SQs: Resource Utilization (now called Life Cycle Efficiency) Dependability, and Flexibility (now called Changeability). Dependability also needs Maintainability to relate Reliability to Availability; and Changeability also needs Maintainability to address new system challenges and opportunities. This paper summarizes resulting changes in the SQ ontology, and also provides examples of Maintainability need and use, quantitative relations where available, and summaries and references on improved practices.