This figure shows two cells, oriented horizontally, from a rectangular 2-D grid. The grid points along the bottom are labeled (i,j), (i+1,j), and (i+2,j). Those along the top are labeled (i,j+1), (i+1,j+1), and (i+2,j+1). The center of the left cell is thus labeled (i,j), and the center of the right cell is labeled (i+1,j).