  In this article, certain indecomposable Virasoro modules are studied.
Specifically, the Virasoro mode L_0 is assumed to be non-diagonalisable,
possessing Jordan blocks of rank two. Moreover, the module is further assumed
to have a highest weight submodule, the "left module", and that the quotient by
this submodule yields another highest weight module, the "right module". Such
modules, which have been called staggered, have appeared repeatedly in the
logarithmic conformal field theory literature, but their theory has not been
explored in full generality. Here, such a theory is developed for the Virasoro
algebra using rather elementary techniques. The focus centres on two different
but related questions typically encountered in practical studies: How can one
identify a given staggered module, and how can one demonstrate the existence of
a proposed staggered module. The text is liberally peppered throughout with
examples illustrating the general concepts. These have been carefully chosen
for their physical relevance or for the novel features they exhibit.
