When I take 5 and add 6...I get 11, but when I take 6 and 7...I get 1. What am I?

RFY 7
When I take 5 and add 6...I get 11, but when I take 6 and 7...I get 1. What am I?
When I take 5 and add 6...I get 11, but when I take 6 and 7...I get 1. What am I?