Many people, when writing the number 7, still put a small dash in the middle of the number. Officially, this little trace does not exist, as we can see on computer keyboards or calculators. But what is the origin of this custom?
To better understand the different spellings of number 7, it should be remembered that our system originates from the Indo-Arabic system. There is a theory that the format of these numbers is related to the number of angles they have, as shown in the figure below (1 has 1 angle, 2 has 2 angles…). Thus, the trait would be necessary to perform differentiation.
The dash seven is still a feature used in schools so that students in the early grades can differentiate their form from writing number 1. The same feature is used in computer-related activities to guide typists in differentiating "zero" from to the letter "O". At zero, an internal stroke is placed diagonally.