Conditional probability of suit distribution
Known suit: a suit where the distribution is known.
Psuedo suit: is a suit where you think you figured out the distribution (based on the bidding).
If the distrubution of a suit between east west is known than this has consequences for the other suits. The below situation could be north-south are in four hearts holding 8 hearts together, after a weak two in spades from west. The known suit is spades and the key suit is hearts.