The C on the Card, Concealed or Closed and Definition (Article ID 144)
Abstract: We often hear players refer to the letter “c” displayed to the right of a hand on the NMJL card as a “closed” hand. The proper term is a “concealed” hand however, there is no penalty for using the term “closed.” Incidentally, some players refer to the “x” hands as “open” hand . The property term for this type of hand is “exposed”. It is acceptable to use the term closed for concealed and open for exposed with no penalty incurred.
A concealed hand is one whereby you cannot call a tile to expose. However, you can call a tile, if that tile gives you mah jongg. An exposed hand is one whereby you can call a tile at anytime (even if it is not your turn) during the game to expose a pung, kong, quint or sextet.
Question from a player: Sometimes, I hear players refer to a hand with a “c” to the right of it as a closed hand. Is it okay to say closed, or do I have to say concealed? I noticed on the back of the card it shows, concealed.
Resources: Mah Jongg Made Easy, pg. 12, Choosing a Hand, Back of National Mah Jongg League Card.