There is an old saying: "A knight can never gain a tempo." Ever hear it? Did you ever wonder what it means?
Well, let us look at the knight for a minute. It always moves like an "L". It can jump intervening men. It captures the same way that it moves. The only difference between that and the bishop is that the knight can do more. The bishop cannot jump. If you put the knight on a black square, its next move will put it on a white square. The knight always alternates the color square it moves to.
Since the knight always alternates square color as it moves, every other move is to a white square if it starts on black. So, if you want to check the opposing king stuck shuttling between, say, g8 and h8, you have to be able to go to a white square when the king goes to the dark square. That means that your knight has to get to f7 (a white square) when the king gets to h8 (a black square.) Another example is the knight on g5 and the black king on g8. If it is white to move, he wants to have it be black to move so he can check on f7. Therefore white will have to move his knight away from g5. The only problem is that any square he goes to will be white. That means that on the next move black will move his king to h8, a black square. Now white will move his knight again, this time to a black square while the black king will go to g8. No matter what white does, he cannot move his knight to g5 in such a way to get it to be black's move with the king on g8. In other words, since white cannot "lose a tempo", he cannot accomplish the task. By the same token, white cannot "gain a tempo" either. It turns out that the knight, by alternating the color square it goes to every move, cannot gain or lose a tempo. This is a very important fact in many endgames.
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