«sign." The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. Answers.com 13 May. 2009. http://www.answers.com/topic/sign
«sign." The Concise Oxford Dictionary of Literary Terms. Oxford University Press, 2001, 2004. Answers.com 12 May. 2009. http://www.answers.com/topic/sign