  Boring is an operation which converts a knot or two-component link in a
3--manifold into another knot or two-component link. It generalizes rational
tangle replacement and can be described as a type of 2--handle attachment.
Sutured manifold theory is used to study the existence of essential spheres and
planar surfaces in the exteriors of knots and links obtained by boring a split
link. It is shown, for example, that if the boring operation is complicated
enough, a split link or unknot cannot be obtained by boring a split link.
Particular attention is paid to rational tangle replacement. If a knot is
obtained by rational tangle replacement on a split link, and a few minor
conditions are satisfied, the number of boundary components of a meridional
planar surface is bounded below by a number depending on the distance of the
rational tangle replacement. This result is used to give new proofs of two
results of Eudave-Mu\~noz and Scharlemann's band sum theorem.
