  We develop a recipe which allows one to recursively and uniquely decouple the
large scale from the small scale in mass matrices of the seesaw type, up to any
order in the inverse of the large scale. Our method allows one to calculate the
mass matrix of the light neutrinos with arbitrary precision. The same method
can be applied in the case of quark mass matrices in an extension of the
Standard Model with vector-like quarks which have mass terms at a scale much
higher than the electroweak scale.
