Thread:GingerQueen1/@comment-24577221-20171022024739/@comment-24577221-20171114044404

To reciprocate is to go back and forth. In the context of promises and agreements, it refers to something like an exchange of favours. "If your country gives our citizens preferential treatment when they travel to your country, we will do the same thing for yours."

"I promise that if you give me a key, I will give you one back" would be a reciprocal agreement. But it can't work, because you have no idea who gave you a key.