un ‧ writ ‧ ten /ʌnˈrɪtn/ BrE AmE adjective
[ Word Family: noun : ↑ writer , ↑ writing , ↑ rewrite ; verb : ↑ write , ↑ rewrite ; adjective : ↑ written ≠ ↑ unwritten ]
an unwritten rule, law, agreement etc is one that everyone knows about although it is not official:
unwritten rules of social behaviour