9 Taking pieces
When a move positions a piece so that on its next move it can capture an opposing piece, that is called an attack. One particular attack is the attack on the king. It is called a check.
If when checked the king cannot escape then it has been (check)mated. For that reason the king is the only piece which cannot be captured.
If a piece takes another one of the same type and the opponent retakes, we call that an exchange. The material balance remains unaltered, thus the idea of an exchange. The more pieces are exchanged, the nearer we get to the endgame. Here the queens are exchanged.