Related Work
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The development of annotation schemes has become an area of computational linguistics development in its own right, often separate from machine learning applications. Some projects began as strictly linguistic projects that were later adapted for computational linguistics. When an annotation scheme is consistent and well developed, its subsequent application to NLP systems is most effective. For example, the syntactic annotation of parse trees in the Penn Treebank [@Marcus:93] had a tremendous effect on parsing and on Natural Language Processing in general.

In the case of semantic annotations, each tends to have its unique area of focus. While the labeling conventions may differ, a layer of modality annotation over verb role annotation, for example, can have a complementary effect of providing more information, rather than being viewed as a competing scheme. We review some of the major semantic annotation efforts below.

Propbank [@Palmer:2005] is a set of annotations of predicate-argument structure over parse trees. First annotated as an overlay to the Penn Treebank, Propbank annotation now exists for other corpora. Propbank annotation aims to answer the question [*Who did what to whom?*]{} for individual predicates. It is tightly coupled with the behavior of individual verbs. FrameNet [@Baker:1998], a 