The ancient Roman way of writing numbers. Likely because of the elegant style, it is still usually recognized and understood by everyday people (at least from 1 to 10), as well as frequently used on clocks.
1 = I
2 = II
3 = III
4 = IV (one below 5)
5 = V
6 = VI (one above 5)
7 = VII (two above 5)
8 = VIII
9 = IX
10 = X (can be remembered by how 2 V's form an X; 5*2=10)
