If the maxillary canine is denoted as 23, which of the following tooth numbering systems was used?
The FDI two-digit system was introduced in 1970 by the Federation Dentaire Internationale (now known as the FDI World Dental Federation). It is a wholly digital system of tooth notation that makes visual sense, cognitive sense, and computer sense. This notation is also known as ISO-3950 notation.
FDI World Dental Federation notation is widely used by dentists internationally to associate information to a specific tooth. 
Orientation of the chart is traditionally "dentist's view", i.e. patient's right corresponds to notation chart left. The designations "left" and "right" on the chart, however, nonetheless correspond to the patient's left and right, respectively.
Palmer system
Zsigmondy system
Universal system
FDI system