Each I is a finger, V is the thumb and index. X is crossing your index and middle finger.
Masters in classics here. They literally are, even the number 4. Look at a watch with Roman numerals and notice that the 4 is “IIII”.
I = 1 finger
…
V = open hand
…
X = two open hands next to each other
Edit to add:
Why do you get “L” for 50 and “C” for 100? The C “centum” was written with two reversed “L” shapes. Take away half of that, and you are left with L.
Why do you get “M” for 1,000 and “D” for 500? Same reason–doubled up reversed D shapes. Take away half and you’re left with a D.
IV and IX don’t make sense when it could be done more intuitively by IIII and VIIII.
Fewer axe marks.
IIII = 4 IV = 3
IX = 3 VIIII = 6
Sure, but OP is talking about fingers as representation, not markings. Your point is exactly why they did it in writing.
One of my undergrad professors said that they look as such because I, V and X can be easily marked using axes.
That makes sense
