The idea of choosing the layout of the pieces on the back row at random. The point is to avoid opening knowledge. White picks the pieces out of a bag and places them left to right; Black places his pieces on the corresponding squares. Alternatively, each player places his own pieces at random, producing two different arrays. The idea has been tried several times in the last 200 years, but has never caught on. There are problems. Usually special rules are needed for the king (castling?) and the bishops (no two bishops on the same colour squares). More serious is the great difficulty players experience in getting the pieces to work together.