The Pawn

The smallest soldier has the simplest marching order: a pawn moves one square straight forward. Not backward, not sideways. Forward, one square, and that is the whole move:

The white pawn on e4 has exactly one move: forward to e5.

Forward means toward the enemy, so the two armies march in opposite directions. White's pawns walk up the board, toward rank 8. Black's pawns walk down, toward rank 1:

Black's pawns march the other way: this one goes from d5 down to d4.

Never backward is the rule that gives pawns their character. A rook that wanders can wander home; a pawn that steps forward has stepped forward forever. Chess players treat every pawn move with respect for exactly that reason: it is the one kind of move you can never undo.