The circumstances, if any, that permit non-uniform, or differentiated, treaty interpretation are difficult to define. Generally, a differentiated approach stands in tension with the Vienna Convention's rules of interpretation, which apply a methodology based on plain meaning to all treaties. Yet courts, states, and scholars widely accept the notion that some treaties warrant special interpretive rules. Thus far, however, efforts to justify differentiated treaty interpretation on the grounds of subject matter or treaty purpose have proven inadequate. A more promising avenue is the examination of the objective characteristics shared within a treaty type. One such characteristic, I argue, is the treaty's degree of completeness. Specifically, all else being equal, standalone instruments call for less reliance upon extrinsic materials; interstitial instruments demand more.Applying this insight to the tax treaty context, this Article argues that such instruments should not be viewed as complete; consequently, reference to plain meaning or even the treaty parties' mutual intent is often incoherent. Specifically, I contend that tax treaties are jurisdictional overlays to the parties' tax systems and substantially rely upon domestic law. Tax treaties also are not heavily negotiated and instead borrow from concepts that are embedded in model treaties, domestic law, and other international instruments. The highly complex nature of tax law and the factual situations to which it applies, the connection between revenue collection and state sovereignty, and the necessity to combat tax abuse retrospectively further explain the interstitial nature of treaties. Courts are thus justified in relying upon extrinsic, and at times unilateral, materials in the interpretation of tax treaties
