[chrismorgan]

… and there are plenty of types of real-world containers that are defined by what they can contain, by their internal rather than external dimensions. Especially capacity in litres.

Take bicycle tyres. The most popular naming scheme in Australia approximates the external diameter in inches, which is objectively problematic. The technically vastly superior naming scheme endorsed by ISO measures the rim diameter, which is the far more important measurement for determining compatibility, in millimetres. As it stands, each inch measurement corresponds to at least two incompatible sizes, depending on the tyre’s width: for example, 16″ tyres are normally 305mm (e.g. 16×2¼″), but there’s also the rarer 349mm (e.g. 16×1⅜″) commonly used in recumbents.