Secondary discourse relations or 'secedges' are ordinary discourse relations which remain unexpressed in the primary discourse parse of a document, either because they break tree structure, or because they are deemed less important than the relations expressed by the primary tree (hence 'secondary').
A secedge can apply between any two nodes which already exist in an RST tree in either direction, including both EDUs and complex, non-terminal units.
Additional units cannot be added to the tree to accommodate secondary relation argument spans!
A secedge can be added to a tree if and only if:
Secondary edges are not:
Secondary edges are:
As justification for the existence of a secedge we require a discourse marker (DM) which is indicative of that relation, but which is not associated with any relation in the tree.
For the definition of DMs we follow the Penn Discourse Treebank's guidelines in limiting items to:
DMs are further required to signal a relational structure between two argument spans. As a result, attitude markers or stance adverbials such as 'fortunately' do not qualify as DMs in the connective sense (though they may be non-DM signals - see Signalling Annotation).
A complete list of DMs is available from the PDTB annotation manual. The top 10 items for each class are repeated here for illustration:
We allow one secondary relation between each pair of nodes in each direction. As a result, the maximum number of relations between a pair of nodes is 3: a possible primary relation, and one secedge in each direction.
A single DM can only be used to support a single relation, i.e. we cannot create two secedges unless we have two distinct DMs. However DMs may overlap or even be discontinuous in some cases (e.g. either... or; not only... but also).