Magnetic leakage flux in transformer

The flux produced in the primary winding doesn't link fully with secondary winding a part of it leaked by passing through air rather than core.This flux which leaks is called leakage flux.
      In order to reduce leakage flux half of primary and secondary winding were placed concentrically or side by side in each limb of transformer.The primary and secondary winding were always interleaved to reduce leakage flux.