Short answer: the letter is part of the hand, not a preference. Every line on the NMJL card ends in an X or a C, and that letter is a term of the contract. On an X line you may pung or kong a discard and lay the group face up on top of your rack. On a C line nothing may be exposed: apart from the winning tile itself, all fourteen tiles have to come off the wall. Expose once and the C version of that line is gone permanently, along with the higher value printed beside it.
Why the card prices the two letters differently
American mahjong has no chows. You cannot call a tile to complete a sequence — when the card asks for a run, you draw every tile of it yourself. With that shortcut unavailable, there are only two ways to build a hand: take tiles from other players' discards, or wait for the wall to give them to you.
X pays you in speed. Every tile you call is one you did not have to draw. You pay for it in information, because the called group sits face up on your rack for the rest of the hand. Given that the whole set is 152 tiles — 36 in each of three suits, 16 winds, 12 dragons, 8 flowers and 8 jokers — and only four copies of any tile exist, buying the fourth one from a discard is often the difference between a live hand and a dead one.
C pays you in points and in silence. These lines carry higher values precisely because they are harder: you draw thirteen of your fourteen tiles yourself while three opponents throw away tiles you are forbidden to touch. What you get in return, besides the points, is a rack nobody else can read. That matters more here than beginners expect, because most card lines are built from pungs, kongs, quints and matching numbers across suits, all of which become countable the moment one group is exposed.
What each letter actually permits
| X — exposed | C — concealed | |
|---|---|---|
| Pung or kong a discard | Yes | No — dead hand |
| Add a called tile to an exposure you already have | Yes | No |
| Win on another player's discard | Yes | Yes — the 14th tile is never an exposure |
| Self-draw and keep a concealed kong | Yes | Yes |
| Exchange a joker on your own turn | Yes | Yes, after you draw |
| Point value on the card | Standard | Higher |
Two rows cause most of the confusion. A concealed kong made entirely from tiles you drew does not break concealment — nobody but you has seen anything. And a joker exchange is not calling a discard: you lift the joker out of an opponent's exposed group and hand them the real tile, and a C hand may still do it. The order is pick, rack, then exchange; exchanging before you draw is its own dead hand.
The fork, at the table
The Charleston is over and your rack holds two 7-dots, three 3-craks, three red dragons and a scattering of tiles you intend to pass off. You check the card and find two candidate lines with the same groups: one printed X at 30 points, one printed C at 50.
Two turns later the player across from you discards a 7-dot.
If you call it, three 7-dots go face up on top of your rack and the X line is alive. So is everyone's picture of your hand. The C line is dead — not damaged, dead — because concealment is binary and you have now broken it. Nothing you draw for the rest of the hand puts those three tiles back in your rack.
If you let it go, both lines are still available and you need to draw that third 7-dot yourself, which you may never do. Six turns later you might draw it anyway. On the X path that tile arrives too late to matter, and your hand is already declared. On the C path it arrives exactly on time, at the higher number.
Neither choice is "correct" in the abstract. What decides it is which letter was printed on the line you actually intend to win on — and that has to be settled before someone throws a tile you want, ideally while you are still sorting at the end of the Charleston.
Three mistakes that cost hands
- Calling first, reading the card second. The call cannot be undone. If the line you were quietly building turns out to be marked C, your hand dies the instant the group touches the rack. In American mahjong you do not declare yourself dead; the other players judge it from what you have exposed. Read the card before you commit, not after.
- Believing C means you must self-draw the winning tile. You do not. The National Mah Jongg League is explicit that the fourteenth tile may be thrown by another player or picked by the player who is set for Mah Jongg, whether playing an exposed or a concealed hand. The tile you win on never counts as an exposure, on any line.
- Mixing lines because they look similar. Exposing for one pattern does not entitle you to finish on the pattern next to it. A winning hand matches one line exactly — tiles, suits, group sizes and letter. The mistake usually surfaces at the moment of declaring, which puts the hand on the dead hands page instead of the scoring table.
One line to remember
X buys speed by selling information; C buys points by buying patience — and you are charged for the choice before your first call, not after. The full list of what each letter permits is on the concealed hands page.