9 is the 'zero' too.
Casting out 9s - when you are getting the digital root/sum of a number you can remove the 9's, or add 9's indefinitely and still receive the same sum.
18392
1+8+3+9+2 = 23 = 5
183x2
1+8+3+0+2 = 14 = 5
You can even take out multiple numbers that add up to nine
1245
remove the (4 and 5) 1+2+0+0 = 3
1+2+4+5 = 12 = 3
Now try adding 9's to a number and it will stay the same.
1488 = 21 = 3
91488999 = 57 = 12 = 3