(CNN) No matter how bruising the race for the White House may have been, the winners and losers show immense grace when it comes to transitions in the American presidency. Nothing makes that point more strongly than these letters outgoing presidents have left for the incoming occupants of the White House.

Barack Obama to Donald Trump

"Congratulations on a remarkable run. Millions have placed their hopes in you, and all of us, regardless of party, should hope for expanded prosperity and security during your tenure.

This is a unique office, without a clear blueprint for success, so I don't know that any advice from me will be particularly helpful. Still, let me offer a few reflections from the past 8 years.

First, we've both been blessed, in different ways, with great good fortune. Not everyone is so lucky. It's up to us to do everything we can (to) build more ladders of success for every child and family that's willing to work hard.

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