This software is a waste of money. I love playing UniSudoku, but I was ready a for bigger challenge. So, I bought 3Doku. In my first puzzle, I ran into a part of the puzzle that was impossible to solve. Based upon the numbers given in the cells by the program at the beginning of the puzzle, a particular 3 x 3 square could not accept the number 7 in any of the available spaces. This renders that 3 x 3 square unsolvable and thus the entire plane and the entire puzzle unsolvable. When writing to tech support, the only response I received was that my problem was impossible, I must have done something wrong. This problem was based upon the cell values given by the program, not me. Yet, they would not answer follow-up e-mails or requests to refund my money. Do not buy. Terrible support. Useless software.