Usage patterns
〈事〉に開きがあるthere is a gap/difference in something
There is a big gap between their opinions.
〈物〉を開きにするto split and dry (a fish)
I split open the saury and dried it.
開きを縮めるto narrow a gap or difference
The players kept training to narrow the gap in their skill levels.
Expressions
アジの開きbutterflied and dried horse mackerel お開きにするbreak up a meeting; call it a night