מקל ספירה – הבדלי גרסאות

* [http://www.nationalarchives.gov.uk/images/museum/enlarge/6.jpg Photo of Medieval Exchequer Tallies]

* [http://vilda.alaska.edu/cgi-bincdm/viewer.exe?CISOROOT=singleitem/collection/cdmg21&CISOPTR=/id/848&CISORESTMP=&CISOVIEWTMP=/rec/3 Messenger Sticks (Inuit)‎]

* [http://www.nationalgeographic.com/inca/inca_culture_3.html Inca khipus]

* [http://www.fas.harvard.edu/home/news_and_events/releases/khipu_08112005.html khipus as ledgers]

* [http://www.yale.edu/lawweb/avalon/medieval/excheq.htm The Dialogue Concerning the Exchequer]