  We develop a technique that solders the dual aspects of some symmetry. Using
this technique it is possible to combine two theories with such symmetries to
yield a new effective theory. Some applications in two and three dimensional
bosonisation are discussed. In particular, it is shown that two apparently
independent three dimensional massive Thirring models with same coupling but
opposite mass signatures, in the long wavelegth limit, combine by the process
of bosonisation and soldering to yield an effective massive Maxwell theory.
Similar features also hold for quantum electrodynamics in three dimensions. We
also provide a systematic derivation of duality symmetric actions and show that
the soldering mechanism leads to a master action which is duality invariant
under a bigger set of symmetries than is usually envisaged. The concept of
duality swapping is introduced and its implications are analysed. The example
of electromagnetic duality is discussed in details.
