May 2011 edit
Welcome to Wikipedia. Everyone is welcome to contribute to the encyclopedia, but when you add or change content, as you did to the article Abacus, please cite a reliable source for the content of your edit. This helps maintain our policy of verifiability. See Wikipedia:Citing sources for how to cite sources, and the welcome page to learn more about contributing to this encyclopedia. Thank you. NeilN talk to me 22:00, 22 May 2011 (UTC)