Wikipedia
Finger-counting, or dactylonomy, is the act of counting along one's fingers. Though marginalized in modern societies by Arabic numerals, formerly different systems flourished in many cultures, including educated methods far more sophisticated than the one-by-one finger count taught today in preschool education.
Finger-counting can also serve as a form of manual communication, particularly in marketplace trading – including hand signaling during open outcry in floor trading – and also in games such as morra.
Finger-counting varies between cultures and over time, and is studied by ethnomathematics. Cultural differences in counting are sometimes used as a shibboleth, particularly to distinguish nationalities in war time. These form a plot point in the film Inglourious Basterds, by Quentin Tarantino, and in the novel Pi in the Sky, by John D. Barrow.
A person indicating a numeral to another will hold up their fingers to signal the specific number. For example, people from North America and the United Kingdom will raise their index, middle, and ring fingers vertically to signal the number 3.
For Continental Europeans, the thumb represents the first digit to be counted (number 1), as opposed to the index finger in North America and the UK. The index finger is number 2 through to the little finger as number 5. Fingers are generally extended while counting, beginning at the thumb and finishing at the little finger. For example, Europeans would use their thumb, index, and middle fingers to express the number 3, whereas in North America and the UK they would use their index, middle, and ring fingers.
Finger-counting systems in use in many regions of Asia allow the counting to 12 by using a single hand. The thumb acts as a pointer touching the three finger bones of each finger in turn, starting with the outermost bone of the little finger. One hand is used to count numbers up to 12. The other hand is used to display the number of completed base-12s. This continues until twelve dozen is reached, therefore 144 is counted.
Chinese number gestures count up to 10 but can exhibit some regional differences.
In Japan counting for oneself begins with the palm of one hand open. Like in East Slavic countries, the thumb represents number 1; the little finger is number 5. Digits are folded inwards while counting, starting with the thumb. A closed palm indicates number 5. By reversing the action, number 6 is indicated by an extended thumb. A return to an open palm signals the number 10. However to indicate numerals to others, the hand is used in the same manner as an English speaker. The index finger becomes number 1; the thumb now represents number 5. For numbers above five, the appropriate number of fingers from the other hand are placed against the palm. For example, number 7 is represented by the index and middle finger pressed against the palm of the open hand. Number 10 is displayed by presenting both hands open with outward palms.