As we before observed, the Pawn is the only man which captures in a
direction different from his line of march. Suppose, at the opening of
the game, White begins by playing King's Pawn to King's fourth square
(see the article on Notation), Black may reply in the same manner with
King's Pawn to King's fourth square, and neither Pawn can do more than
remain an obstruction to the onward march of the other, but if Black
answer instead with King's Bishop's Pawn to Bishop's fourth, or as in
the diagram, with Queen's Pawn to Queen's fourth, then White, if he
choose, may take the adverse Pawn from the board and place his own in
its stead.

No. 7.

BLACK.
+---+---+---+---+---+---+---+---+
| R*| N*| B*| Q*| K*| B*| N*| R*|
+---+---+---+---+---+---+---+---+
| P*| P*| P*| | P*| P*| P*| P*|
+---+---+---+---+---+---+---+---+
| | | | | | | | |
+---+---+---+---+---+---+---+---+
| | | | P*| | | | |
+---+---+---+---+---+---+---+---+
| | | | | P | | | |
+---+---+---+---+---+---+---+---+
| | | | | | | | |
+---+---+---+---+---+---+---+---+
| P | P | P | P | | P | P | P |
+---+---+---+---+---+---+---+---+