http://www.sizes.com/numbers/roman_numerals.htm if you're having trouble believing that the pope has the roman numerals 666 on his mitre.....please scroll down to the bottom of this webpage and read....how V is the transformation of U.

V=5
I=1
C=100
A=0
R=0
I=1
U=5
S=0

F=0
I=1
L=50
I=1
I=1

D=500
E=0
I=1

500+100+50+5+5+1+1+1+1+1+1= 666