Many of the terms the Constitution uses to define Commonwealth legislative powers are legal terms or at least terms well-known to the law with legally defined meanings in 1900. The ordinary meaning of some of these terms is dependent on, and derived from, their legal meaning. Therefore, it is reasonable to assume that their constitutional meaning is consistent with their legal meaning. This poses a problem: what is the legal meaning of a legal term? One answer may be that the legal terms are shorthand for the bundle of legal rights and duties and practices associated with that term. Interpreting some Commonwealth power as powers over evolving, contested practices allows the scope of those powers to evolve while the basic meaning, that of a power over an evolving legal or social practice, remains unchanged. Such an approach to interpretation is most relevant to the problem of interpreting powers over areas of law, where it enables workable limits to be placed on powers which, if interpreted in other ways, are either so narrow that they allow no innovative legislation or have no effective boundaries. It is also relevant to powers over contested social practices such as marriage and international relations, which the framers knew would change over time. Although not relevant to the interpretation of all powers, some of which cannot be interpreted as powers over contested social practices, it is an interpretive approach which does allow the content of some powers to change, consistently with the idea that the Constitution, as a document, has an unchanging meaning.