Nobel, Philip. Sixteen Acres: Architecture and the Outrageous Struggle for the Future of Ground Zero. Macmillan. 2005年: 54页. ISBN 0805080023. NIST NCSTAR
Joan Bybee, Revere Perkins, William Pagliuca,"The Evolution of Grammar: Tense, Aspect, and Modality in the Languages ofthe World(語法的演化--世界語言的時,體和情態)"[6]