Chess Guide for the Intermediate Player/Tactics for beginners
Values is an approximately number for evaluate and analysis the function. If you are "up a material", the value will be positive; vice versa.
Let's say the pawn is worth 1 unit. Then, bishop and knight(also known as "minor pieces") worth 3 unit, rook worth 5 and queen worth 9. (They are called "major pieces".) King, of course, worth ∞ unit, because if you lose your King, you lose the game.
There's two variation of this value system: one says minor pieces worth 3½ units, another is more complicated. We'll see the second one in the "Engine Programming" chapter because it is usually used in chess engines.
By the way, units is sometimes called "points".
Exchange is the simplest tactic. It means capture a defended piece. Because that piece is defended your piece will immediately get recaptured by the defender. Exchange is sometimes called "trade".
We can use values in order to decide to exchange or not. What's the value of the piece that you captured? What about the pieces that he captured? For example, you should NOT use a queen to capture a defended pawn(or "trade a queen for a pawn" because a queen worth 9 points but a pawn only worth 1.
Sometimes exchange can be complex, because a piece can has multiple defender and attacker so after the first attacker is lost, the second attacker can continue. This is called complex exchange. Calculate carefully before a complex exchange. A way to create complex exchange is to build "batteries" which means line up multiplex long-ranged pieces in a row so they can make breakthrough on the piece it is aiming even though that piece is defended.
Trade Example 1: an "even" trade
Now we are going to look at an even trade which happens in opening. An even trade is a trade where you didn't gain any advantage on values, but also didn't lost any.