The roman numerals are easy to understand but what was
the logic behind the Phoenician numbers?
It's all about angles ! It's the number of angles. If one writes the numbers down (see below) on a piece of paper in their older forms, one quickly sees why. I have marked the angles with "o"s. No 1 has one angle. No 2 has two angles. No 3 has three angles. etc. and "O" has no angles
Interesting, isn't it...?