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Keyes, John (22 August 2003). "KarTrak". John Keyes Boston photoblogger. Images from Boston, New England, and beyond. John Keyes. Archived from the original on 10 March 2014. Retrieved 26 May 2013.
"Barcode". iWatch Systems. 2 May 2011. Archived from the original on 9 January 2012. Retrieved 28 November 2011.
Oberfield, Craig. "QNotes Barcode System". US Patented #5296688. Quick Notes Inc. Archived from the original on 31 December 2012. Retrieved 15 December 2012.
GS1 Bar Code Verification Best Practice work team (May 2009). "GS1 Bar Code Verification for Linear Symbols"(PDF). Global Standards 1. 4 (3): 23–32. Archived(PDF) from the original on 27 September 2011. Retrieved 2 August 2011.{{cite journal}}: CS1 maint: numeric names: authors list (link)