Chess Knight Move

One day long ago, a man and woman got married. The man told the woman that there would always be a box under the bed but to never look into it. So they were married for 40 years and the woman never looked in the box. On the morning of their 40th anniversary, the wife looked in the box. In the box, there was about 300 dollars in small bills, and 3 empty beer bottles. At dinner that evening, the woman just had to ask. So she did, she asked "what are those beer bottles for, you know, in the box under the bed?" The man said, oh no, you looked. OK, Every time I've been unfaithful to you, I chugged a beer and put it in the box.
The wife says, well for forty years, that's not so bad. At night, the woman was having a bad night, she could not get to sleep, something was bugging her. Then she remembered. She shook awake her husband and asked, what was the money for, though. The guy says, what? The lady says, you know, the money in the box.
The guy says, well, every time the box filled up, I took it in and got money for the bottles.