1. IMPACT Project. Improving Access to Text. http://www.impact-project.eu
2. Oxford English Dictionary. The Oxford English Dictionary, 2nd edn. 1989. OED Online, Oxford University Press, Oxford (2000). http://dictionary.oed.com
3. Oxford English Dictionary, Writing the OED (2010). http://www.oed.com/about/writing/
4. Google books (2011). http://books.google.com/
5. Project gutenberg (2011). http://www.gutenberg.org/