Interoperability is a common understanding of the meaning of data between a sending and receiving computer system [1]. The level of interoperability required varies with application needs. The specification of data in enough detail to create a common shared meaning between organizations is a complex task as systems work within organizational and human factors contexts as well as having specific technical requirements. Aequus communis sententia translates from Latin to the "level of common meaning." In this manuscript, we define an Ontology of Interoperability. The scale asks reviewers of a specification to define it's level in terms of Syntactic, Semantic and Pragmatic Interoperability. We tested the scale by having five medical Informaticians rate a set of ANSI standard specifications and we report the inter-rater variability of the interoperability rating scheme. We learned that some elements of the scale presented more difficulty for our reviewers and based on our findings we present a final version of the interoperability scale in our discussion. Our interoperability rating ontology has high inter-rater reliability and is a relatively simple mechanism for comparing the levels of interoperability afforded by different specifications or the same specification over multiple versions.