  We introduce and study a class of compact 4-manifolds with boundary that we
call protocorks. Any exotic pair of simply connected closed 4-manifolds is
related by a protocork twist, moreover, any cork is supported by a protocork.
We prove a theorem on the relative Seiberg-Witten invariants of a protocork
before and after twisting and a splitting theorem on the Floer homology of
protocork boundaries. As a corollary we improve a theorem by Morgan and
Szab\'{o} regarding the variation of Seiberg-Witten invariants with an upper
bound which depends only on the topology of the data. Moreover, we generalize
the result that only the reduced Floer homology of a cork boundary contributes
to the variation of the Seiberg-Witten invariants under a cork twist to more
general cut and paste operations where the pieces involved are $1$-connected
and homeomorphic relative to the boundary.
