# Crow's foot notation--what is the difference between “one” and “one-and-only-one” regarding relationship cardinalities?

I'm trying to figure out the difference between the single-bar and double-bar cardinality indicators as in the figure below:

Some sources seem to use one notation while others use the other. I can't tell if they actually refer to different kinds of relationships of if the difference is only in notation and they mean the same thing.

• Possible duplicate of: stackoverflow.com/questions/33781451/… – Doug Deden Jul 2 '19 at 18:51
• I'm not seeing a consensus in the answers to the first question but u/dosenmatter suggests that "one" really means {0, 1} whereas "one-and-only-one" means {1}. This would make sense to me even though it results in the kind of strange one =/= {1}. It also seems to indicate that "one" is equivalent to "zero-or-one". I guess the single-bar cardinality indicator is ambiguous, although as u/Christophe's answer in the second question points out, this can sometimes be useful. – ahh_real_numbers Jul 3 '19 at 17:00