Cross-sum or number crossword puzzles, also known as Cross Sum, Cross Addition, and Kakuro, have many names. They are beautiful number puzzles often described as a mix between Sudoku and crossword puzzles. These logical challenges were invented in Canada but only gained popularity years later in Japan, where they even became significantly more popular than Sudokus for a time. A large fan base has also developed in Germany, and so we are finally presenting a successor to Volume 5 of this series. Welcome! Is Kakuro the new Sudoku? Not quite, as that never happened, even though there are some similarities. For example, neither originated in Japan. While Sudoku was born in the USA, Kakuro was invented by a Canadian and published in America for decades under the name Cross Sum before it eventually made its way to Japan.