T-duality

From Physics wiki

As a transformation, T-duality exchanges Neumann and Dirichlet boundary conditions.

See also: T-duality (Bosonic string)

Compactification

In string theory, T-duality arises when we consider compactifying one or more spacetime dimensions, giving each the structure , and the overall spacetime the topology of a torus. This is analogous to Kaluza-Klein theory in quantum field theory.