Quick answer
The courtesy pass is an optional exchange of zero to three tiles with the player across from you, made after the Charleston ends and before the first tile is drawn. Each of you names a number and the lower one is used, so either player can make it zero. A joker may not be passed in it, the same as in every other part of the Charleston.
The courtesy pass is the last exchange before play, and the one where you choose how many tiles move. You can take zero tiles. You can take one. It comes after the last Charleston pass, when you have seen every tile the Charleston brought you.
When it happens and with whom
The courtesy pass comes after the Charleston has finished, whether that was after the first round alone or after both, and before the first tile is drawn. It is the last thing that happens before play.
It is across only. You exchange with the player directly opposite, nobody else, which is why it is sometimes called the optional across. The other two players are not part of it and may be doing their own exchange at the same time.
How the number works
Each of you names how many tiles you want to exchange, up to three. The lower number is the one used. If you want three and the player across wants one, you exchange one each.
Zero is a legal answer. If your hand is where you want it, you say zero and nothing moves. That is not rudeness, it is the mechanic working as designed, and it is why this is the only pass in the game where fewer than three tiles can change hands.
The cap is three. If your group plays a different cap, settle it before the deal rather than negotiating it mid-hand.
No jokers, still
The rule against passing a joker runs all the way through the courtesy pass. A joker may never be passed in any part of the Charleston, and the courtesy pass is not an exception.
This is worth knowing precisely because the courtesy pass feels like a separate, gentler thing happening after the rules part is over. It is not. If you are holding a joker you do not want, it stays with you until you can use it or discard it in play.
What to actually move
By this point you have been through three passes, or six if the second Charleston was played, and you have a direction. The tiles worth exchanging are the leftovers of the direction you gave up on, such as the lone honour from a hand you abandoned, the third suit you stopped feeding, or the odd tile that survived because it was never quite worth passing.
Ask for the smallest number that clears those. Naming three when one would do means giving up two tiles you actually wanted, and the player across is under no obligation to send you anything useful.
Think twice before breaking a pair to fill the number. A joker can never be part of a pair, so a paired tile you send away can only be replaced by the real tile.
What you get back
You have no control over what arrives, which is the honest limitation of the exchange. The player across is shedding their own leftovers, and those are not chosen with you in mind.
That cuts both ways and it is the reason to keep the number small. A one-for-one exchange is a cheap look at one more tile. A three-for-three is a real reshuffle of a hand you have already shaped.
Put it into practice
New to the game? Start the course at Lesson 1, Meet the Tiles. The first three lessons are free and need no account.
Already play? Have the coach read a real dealt hand for you, free. It names the direction the hand goes for, what to keep, what to pass, and the reasoning behind each call.
The courtesy pass is for the leftovers of a direction you already abandoned. Mei, our AI coach, reads a dealt hand and says which direction it goes for, what the backup is, and which tiles are doing nothing for either.