An extension of one (irreducible, say) p-adic Galois representation by another is said to be {\em geometrically modular} if it can be realised in the cohomology of an ''open Shimura variety" (such as the complement of a sub-Shimura variety of a Shimura variety.) It is natural to wish to describe the extension classes that are expected to be geometrically modular, or to arise as p-adic limits of geometrically modular representations.