This applet is great for studying the relationships between perimeter and area and length and width of a rectangle. Use the Set Area and Set Perimeter options to hold the area, perimeter, or both constant. Use the Line of Constraint option to draw a line and allow only rectangles with their upper right hand corner on that line. Use the trace option to trace that same corner. The trace is most useful to use along with the Set Area and Set Perimeter options.