expression
good fences make good neighbors; a hedge between keeps friendship green
Proverb meaning that even close friends should maintain courtesy and respect boundaries.
親しき仲にも礼儀ありで、長年の友人にも敬語を使うようにしている。
As the saying goes, good fences make good neighbors; I try to use polite language even with long-time friends.
A Japanese proverb of uncertain origin, combining 親しき (classical attributive form of 親しい, 'close, intimate'), 仲 ('relationship'), にも ('even in'), 礼儀 ('courtesy, manners'), and あり ('exists').