How to play
YAHTZEE was invented by a wealthy Canadian couple to play aboard their yacht. Whenever friends were invited aboard, they were taught how to play the "Yacht" game. Their friends enjoyed the game so much that they all wanted copies of their own.
—History of Hasbro: Yahtzee
Yahtzee is a classic dice game from 1950s that is played with 5 dice and a score sheet. Then players take turns throwing all 5 dice and choosing whether to reroll any number of dice for maximum of 2 rerolls (for 3 rolls total), and scoring points. You can score points for any single row that's empty and which combination your dice make. You have to score a row each turn even if it meant scoring a row for which you had no combination, marking it as 0.
Scoring
You score 1 row each turn in any order. In the Upper Section, you score similar to YATZY —summing together all matching dice. The BONUS, however, is only 35 points.
For 3 of a kind and 4 of a kind, you score the sum of ALL dice (not just the ones that are same). For Full House you score fixed 25 points. Small Straight is any 4 dice in sequence, thus a lot easier than in YATZY, whereas Large Straight is either 1-2-3-4-5 or 2-3-4-5-6 sequence.
Yahtzee
YAHTZEE scores the same as in YATZY, 50 points. However, in YAHTZEE you receive extra 100 bonus points for each additional YAHTZEE you score after the first. If you have already set YAHTZEE row as 0, you do not receive the bonus.
In addition to the bonus, you have to score a row as normal and must score Ones-Sixes row corresponding to the YAHTZEE first. If that row, however, has already been filled, you can score any empty Lower Section row.
Additionally, you can score Full House, Small Straight or Large Straight row for full points without actually having the combination for it. YAHTZEE serves then as a Joker.
Winner
Whoever has the most points when all the rows have been filled after 13 rounds, is the winner.