The ladder mate can be executed by any pieces that are able to control the ranks or the files and most popular will be two rooks, a rook and a queen, or even two queens. Now the idea behind the ladder mate is we want to force our opponent's king into one of the four corners. The four corners being the H file, the A file, the eighth rank, or the first rank. We're going to use our control of the ranks and files. Together, these two pieces are going to force the king into one of the four corners and, ultimately, execute the checkmate.In this position, these two pieces are currently controlling the B file. What that means is black's king is unable to go anywhere on the B file due to the control of the rooks on the B file. So when white is looking for a move, white will leave one rook which controls the B file and it will take the other rook and control the C file, ultimately forcing the king to go, then, to the D file.