The applet presentsat two-dimensional lattice in which two toolbars can be used to change the[br]numbers of the rows and the columns. One can drag each of the vertices of the[br]basic quadrilateral, thus changing its sides, since the areas in each of the[br]rows constitute an arithmetic progression, and similarly the areas of the[br]columns. The common differences of the areas of the rows and the columns are[br]shown on the screen. The screen also shows the connection between the distance[br]between the mid-points of the external quadrilateral and the distance between[br]the mid-points of the corner sub-quadrilateral (left corner top row).