When a defender is running a suit, how can he/she let partner know which cards to keep?
No-trump contract, South to lead:

At this point in the play everyone knows that South is out of ♥️s. What they don’t know is which suit South will lead after running the ♦️s. Will it be a ♣️ or will it be a ♠️? How can we help North decide which ace to keep?
I propose the following simple partnership agreement:
Running your suit by playing highest to lowest says “save the high ranking suit”.
Running your suit mostly from low to high says “save the low ranking suit”.
In the example above South plays ♦️s in the order: 9, 10*, 2, J.
This tells North “Save the low ranking suit–♣️s”.
Perhaps South’s last card is a ♠️ as below:

Now South plays the ♦️s in the order: J, 10, 9, 2.
This tells Partner (North) “Save the high ranking suit–♠️s”.
This situation doesn’t come up that often but when it does you will be ready for it.
*[South played the 10♦️ second because he/she was not sure the 2♦️ was good.]
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