Бројевни систем — разлика између измена

* {{Cite book| ref = harv | author= Hans J. Nissen, P. Damerow, R. Englund, |title=Archaic Bookkeeping|location=|publisher=University of Chicago Press|year=1993|isbn=978-0-226-58659-5|pages=}}.

* {{Cite book| ref = harv | author= Denise Schmandt-Besserat, |title=How Writing Came About|location=|publisher=University of Texas Press|year=1992|isbn=978-0-292-77704-0|pages=}}.

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