  We advocate an account of dualities between physical theories: the basic idea
is that dual theories are isomorphic representations of a common core. We
defend and illustrate this account, which we call a Schema, in relation to
symmetries.
  Overall, the account meshes well with standard treatments of symmetries. But
the distinction between the common core and the dual theories prompts a
distinction between three kinds of symmetry: which we call `stipulated',
`accidental' and `proper'.
