If you score a Yahtzee (meaning that all 5 dice are the same) and that combination has already been used, then a special Joker Rule applies. It doesn’t matter whether you scored 50 points because of an earlier Yahtzee, or whether you at some point selected the combination without a Yahtzee and therefore scored zero points.
According to the Joker rules, the Yahtzee can be used to score Full House (25 points), Small Straight (30 points) or Large Straight (40 points), even though the dice don’t meet the normal requirements for those categories. In these cases the Yahtzee is said to act as a “Joker”.
There are three alternative versions of the Joker Rule:
Forced Joker Rule
This is the official joker rule from Hasbro, the current owner of the game.
If the corresponding upper section combination is unused then it must be used now. So if you for example roll 5 fives and combination 5 is unused, then it is mandatory to select combination 5 and thereby score 25 points.
If the corresponding upper section combination is used, then a lower section combination must be used. The Yahtzee can act as a Joker for the Full House, Small Straight and Large Straight combinations, as mentioned earlier.
If the corresponding upper section combination and all lower section combinations have been used, then an alternative upper section combination must be used (resulting in a score of zero points).
Free Choice Joker Rule
Under this rule, it is not mandatory to use the corresponding upper section combination. But if it is unused and you do not use it, then the Yahtzee cannot be used as a Joker in the lower section. So if you for example roll 5 Threes and combination 3 is unused and you choose to select Full House, then the score will be zero points.
Original Joker Rule
With the Original Joker Rule (the official one from 1956 to 1961), the Yahtzee can never score any points in the upper section, only in the lower section. So if you for example roll 5 Fours and combination 4 is unused and you choose to select that combination, then the score will be zero points.