Nonexiting pieces are labeled 1 through 6 and will be referred to as number pieces. Thousands of problems are possible by placing the pieces on the board in different configurations. The primary objective is to move all of the letter pieces to the center. A secondary objective is to do so in a specified minimum number of moves . Letter pieces never begin in the center and are removed whenever they stop there.
Number pieces can begin or stop in the center, but are never removed.
A move, which is the same for letter and number pieces, is made in steps . For each step, a piece travels left, right, up or down towards another piece, stopping in the next to the piece moved towards. Travel is stopped only by another piece and must always be towards another piece. It must be removed from the board immediately and if it is the last, or only, letter piece, the problem has been solved. However, a letter piece may travel through the center.