Published online by Cambridge University Press: 20 November 2018
A subset   $W$  of a closed manifold
 $W$  of a closed manifold   $M$  is
 $M$  is   $K$ -contractible, where
 $K$ -contractible, where   $K$  is a torus or Klein bottle if the inclusion
 $K$  is a torus or Klein bottle if the inclusion   $W\,\to \,M$  factors homotopically through a map to
 $W\,\to \,M$  factors homotopically through a map to   $K$ . The image of
 $K$ . The image of   ${{\pi }_{1}}\left( W \right)$  (for any base point) is a subgroup of
 ${{\pi }_{1}}\left( W \right)$  (for any base point) is a subgroup of   ${{\pi }_{1}}\left( M \right)$  that is isomorphic to a subgroup of a quotient group of
 ${{\pi }_{1}}\left( M \right)$  that is isomorphic to a subgroup of a quotient group of   ${{\pi }_{1}}\left( K \right)$ . Subsets of
 ${{\pi }_{1}}\left( K \right)$ . Subsets of   $M$  with this latter property are called
 $M$  with this latter property are called   ${{\mathcal{G}}_{K}}$ -contractible. We obtain a list of the closed 3-manifolds that can be covered by two open
 ${{\mathcal{G}}_{K}}$ -contractible. We obtain a list of the closed 3-manifolds that can be covered by two open   ${{\mathcal{G}}_{K}}$ -contractible subsets. This is applied to obtain a list of the possible closed prime 3-manifolds that can be covered by two open
 ${{\mathcal{G}}_{K}}$ -contractible subsets. This is applied to obtain a list of the possible closed prime 3-manifolds that can be covered by two open   $K$ -contractible subsets.
 $K$ -contractible subsets.