Perpetual Check

One way to make a draw in chess is by perpetual check: if one can get to check the king of the opponent forever, the game is a draw. For instance, look to the following position:

White to move. How can white make a draw?

It does not look too good for white: black can mate him in one move, e.g., by moving his queen to a3. However, he can force a draw by perpetual check: he can continue to give check, and the game is a draw.

Note that to have perpetual check, it is necessary that the same positions appear after a while; just giving `a lot of' checks after another isn't sufficient to have perpetual check.