Basic concepts of board evaluation A advantage applies both strategically and tactically. Generally more pieces or an aggregate of more powerful pieces means greater chances of winning. A fundamental strategic and tactical rule is to capture opponent pieces while preserving one's own. Bishops and knights are called minor pieces. A knight is about as valuable as a bishop, but less valuable than a rook. Rooks and the queen are called major pieces. Bishops are usually considered slightly better than knights in open positions, such as toward the end of the game when many of the pieces have been captured, whereas knights have an advantage in closed positions. Having two bishops (the ) is a particularly powerful weapon, especially if the opposing player lacks one or both of their bishops. Three pawns are likely to be more useful than a knight in the endgame, but in the middlegame, a knight is often more powerful. Two minor pieces are stronger than a single rook, and two rooks are slightly stronger than a queen. The bishop on squares of the same color as the player is slightly more valuable in the opening as it can attack the vulnerable f7/f2-square. A rook is more valuable when with another rook or queen; consequently, doubled rooks are worth more than two . One commonly used simple scoring system is: {| class="wikitable" ! Piece !! Value |- | Pawn || align="center" | 1 |- | Knight || align="center" | 3 |- | Bishop || align="center" | 3 |- | Rook || align="center" | 5 |- | Queen || align="center" | 9 |} Under a system like this, giving up a knight or bishop to win a rook ("winning the exchange") is advantageous and is worth about two pawns. This ignores complications such as the current position and freedom of the pieces involved, but it is a good starting point. In an open position, bishops are more valuable than knights (a bishop pair can easily be worth seven points or more in some situations); conversely, in a closed position, bishops are less valuable than knights. A knight in the center of the board that cannot be taken, however, is known as a knight outpost and threatens several fork instances. In such a case, a knight is worth far more than a bishop. Also, many pieces have a partner. By doubling up two knights, two rooks, rook and queen,