Mr. Wilhelm Steinitz, the first world champion is considered the father of positional chess. In simple terms, we can understand his chess thinking systems like this:
Thinking Before Steinitz: Lead in development leads to winning combinations & fierce attacks which lead to material advantages and win. Blitz games are still won in this style at low levels.
Steinitz thinking: Lead/ equality in development shall lead to accumulation of small positional advantages which lead to combinations & fierce attacks leading to material advantage and win.
Thus, Steinitz led the game of chess away from gambling. He could nip the opponent's combinations in the bud itself through his grand plan of accumulation of small advantages, attacked fiercely after gaining sufficient positional lead and kept on winning leaving combinational magicians gasping and spectators bedazzled. This was the birth of competitive play in chess tournaments and concrete planning in chess.
The basis of Steinitz's thinking system was that superior position of our pieces are hard to maintain for long but the superior position and structure of our pawns lasts till end. So, focus in the opening to middle phase shall also be on pawns and pawn structures and not only on pieces alone.
But, what small advantages Steinitz was telling to accumulate.:
1. Keeping connected pawns with us (strong position for us)
2. Creating backward pawns in opponent's wing (medium weakness for opponent)
3. Creating doubled pawns in opponent's wing (Serious weakness for opponent especially isolated doubled pawns)
4. Isolation of opponent's hostile pawns (Medium weakness for opponent)
5. Keeping our majority of queen side pawns to create remote passed pawn advantage in the end (Philidor focused on the king-side pawns but Steinitz on queen-side pawns also)
6. Weakening of the phalanx of hostile pawns especially in the vicinity of the opponent's king
7. Creation of secure advanced posts and posting pieces there
8. Domination of open lines
9. Play in the centre
10. Assault with a queen-side pawn-chain to cramp the enemy (Philidor talked about king-side pawn chains only)
11. Keep the Bishop Pair advantage with you and devoid your opponent from this advantage if possible (exchange his one bishop with your knight)
12. Avoiding all such weaknesses in our camp (unless serious material loss is threatened) and create these positional strengths for us.
Steinitz's Theory of Balance of Positions and the Principle of Attack: Steinitz says that if advantages held by one chess player are compensated by the advantages held by his opponent, then the position is balanced and one shall not attack in these positions. Balanced positions with best play from both sides will again and again lead to balanced positions.
Only, after balance of positions has been disturbed (by a weak move or bad play by opponent), a chess player gets an uncompensated advantage and he shall attack to win.
Where to attack: Steinitz compares the position of opponent to a chain of many links and the attacker must attack the points where the opponent's chain is weakest and must break the opponent's chain. So, follow the path of least resistance in attack. Commonly, the weakest point has always been found to be the opponent's f pawn (f7 & f2 pawns).
Steinitz's Theory of Economy and the Principle of Defence: On the flip side of advantage and attack, Steinitz says that if a player is at a disadvantage, he must defend with the principle of economy. The principle of economy says that grade your weaknesses and improve the worst weakness first and proceed in that order till balance or advantage is achieved. Stability of a position is gauged by its least stable point and so a player must attempt to achieve at every point the same degree of stability.
References: (As the goal of this site is to evolve a grand synthesis, example games are outside its scope unless they illustrate the final theory. Kindly purchase and read these important books for more details and examples for these introductory sections)
1. Lasker's Manual of Chess by Emanuel Lasker, Russel Enterprises, Inc, Milford, CT, USA 2008
2. My Great Predecessors - Part I by Gary Kasparov, Everyman Chess