The Pawn.
Here you see 16 Pawns, therefore all that were in the box, and two Rooks, one Bishop, one Knight, the two Kings besides. At the beginning of the game the White Pawns are placed on the second row and the Black Pawns on the seventh row; thence they move or capture ahead towards the enemy, the White Pawns from below upwards, the Black Pawns in the opposite direction. For instance, Pawn d4 may capture e5 and conversely, because the Pawns, though moving ahead in their file, capture obliquely, always advancing towards the enemy.
The above position shows three immobile Pawns, "blocked" Pawns: g3, g4, f7. Pawn g3 is blocked by g4, because the Pawn does not capture straight ahead but obliquely. The position shows nine Pawns standing on the squares where they stood at the start of the game: a2, b2 e2, f2, h2, a7, b7, c7, f7; they have not moved yet; the other seven Pawns have advanced during the progress of the game. The Pawn d4 has two possible moves: to advance to d5 or to capture e5. The Pawn c3 has only one possible move: to advance to c4.
Now the rule was introduced about four centuries ago that Pawns in their initial position and which are not blocked may advance one or two steps according to the plan of the player. This rule made the game more lively, and therefore the Chess world accepted it in time. For instance, Pawn a2 may advance to a3 or to a4 in one move.
With this rule a difficulty arose. Its object was to accelerate the pace of the Chess events and to add to their variety, but it betrayed sometimes the obvious rights of the opponent. To illustrate this point, observe the two Pawns f2 and g4. The Pawn g4 stands on guard over f3. If f2 advances to f3, g4 can capture it; thus it had been for many centuries; after the introduction of the new rule Pawn f2 could evade Pawn g4 by advancing at once to f4 and could then molest Black unpunished. Naturally, the Pawn g4 on guard felt itself deceived, when the hostile Pawn crept through the advance posts. There were scenes of hot dispute. It could not be the meaning of the innovation to make the advancing Pawn immune. And finally justice was victorious: the Pawn standing on guard was acceded the right of capture, just as if the Pawn trying to slip through had advanced one step only; but the Pawn on guard cannot defer this movement but must execute it without loss of time as an immediate reply to the attempted advance. If, for instance, in the above position White moves f2-f4 Black may answer g4 x f3, thus executing his original intention of capturing the Pawn on f3. This species of capture is named "capture in passing" or, with the French expression, capture "en passant." If the Pawn, after f2-f4, is not immediately captured by g4 "in passing," it stays unmolested on f4 and has thereafter to contend only with the hostile Pawns of the f and e line.
The Pawns only advancing ahead arrive, in advancing row by row finally to the eighth row where according to the rule they would come to a barrier and would be immobile. Should this signify their death? Should they now become useless after having done their duty and fought their way through the ranks of the enemy? That would not be in keeping with justice. Since in a struggle it is honourable to draw upon oneself the fire of the enemy and to do him harm, the Pawn advancing to the last row is rewarded by becoming an "officer" in its army; it is changed for a Queen, Rook, Bishop or Knight, according to the will of the player; it is promoted to a higher rank since officers have much more mobility and value than Pawns.