A two-dimensional lattice can be described completely by two vectors. The applet displays two vectors and the lattice they generate (using a hexagon as the basis). These two vectors may be dragged using the mouse, and the lattice will update accordingly.
The two vectors should be linearly independent. If a = kb (for some real number k), then all the points in the lattice lie on a straight line.
If the two vectors are almost dependent (ie. the angle between them is very small), then the lattice will be very dense. In such cases, the applet will draw no more than approximately 3700 instances of the basis.