  Quality factors like passive voice or sentence length are commonly used in
research and practice to evaluate the quality of natural language requirements
since they indicate defects in requirements artifacts that potentially
propagate to later stages in the development life cycle. However, as a research
community, we still lack a holistic perspective on quality factors. This
inhibits not only a comprehensive understanding of the existing body of
knowledge but also the effective use and evolution of these factors. To this
end, we propose an ontology of quality factors for textual requirements, which
includes (1) a structure framing quality factors and related elements and (2) a
central repository and web interface making these factors publicly accessible
and usable. We contribute the first version of both by applying a rigorous
ontology development method to 105 eligible primary studies and construct a
first version of the repository and interface. We illustrate the usability of
the ontology and invite fellow researchers to a joint community effort to
complete and maintain this knowledge repository. We envision our ontology to
reflect the community's harmonized perception of requirements quality factors,
guide reporting of new quality factors, and provide central access to the
current body of knowledge.
