Instructions: Click on a square to select it, then type a number to put the number in the selected square. Once all of the numbers have been entered, click "Solve" to run the solver, and the solution will appear. If you entered something incorrectly by mistake, or want to try a different puzzle, click "Reset" to delete any numbers that aren't in bold, or "Clear" to clear the entire board.
Simple Operations - Easy: Simple Operations describe the basic rules of the game. Each column, row, and group must contain no more than a single instance of each number. For example, if there is a 5 in one space, another 5 cannot be placed in the same row, column, or group.
Hole Operations - Medium: Hole Operations are those that rely on there only being one place a number could go within a column, row, or group. For example, if a column has three remaining spaces for a 1, 4, and 7, but two of the spaces have a 4 in the same row, then the 4 must go in the third space.
Phantom Operations - Hard: Phantom Operations are a way of translating the constraints of a group into constraints on columns and rows. For example, if there are only two remaining spaces in a group for an 8, and they line up, then the rest of the column or row that they line up with cannot contain an 8.
Crowding Operations - Evil: Crowding Operations are an extension of Hole Operations into multiple spaces. While a hole can only describe a single value in a single space, Crowding Operations describe a set of n values that can fit into n spaces, which therefore prevents those values being used elsewhere in the row, column, or group. For example, if a group has four spaces for a 2, 6, 7, and 9, but two of those spaces only allow 2 and 6, then the other spaces can only allow 7 and 9.