"This little man," said Adolf Schwarz, "taught us all to play chess".

Steinitz started as a dashing attacking player, in the style common at the time, and claimed the World Championship after defeating Anderssen in a match (although the official beginning of the lineal championship began with his match with Zukertort many years later). By the time he played Zukertort, he was playing in a completely different style, and in the development of Steinitz' style we can see the beginnings of modern chess.

How did he teach? Steinitz didn't write books, but he wrote articles, and edited magazines, and wrestled the chess world into a different appreciation of chess. He had a fluent and pungent pen, and, for all his dogmatism on some subjects, he was right more often than he was wrong.

To search for lessons from Steinitz is not a long job: many of his games pioneered and/or showed in clear form so many of the ideas of positional play -- weak pawns, weak squares, the minority attack, and so on. So the examples below could be multiplied many times over.

Opening

Several of the games under Middlegame and Endgame have lessons for the Opening, but let's focus just on that phase first. As a starter, the dangers of castling into an attack when the centre is closed. I always welcome propaganda against Old Stodge...

[Event "London"]
[Site "attack: castling into it"]
[Date "1862.??.??"]
[Round "?"]
[White "Dubois"]
[Black "Steinitz, Wilhelm"]
[Result "0-1"]
[ECO "C50"]
[Annotator "attack: castling into it"]
[PlyCount "78"]
[EventDate "1862.??.??"]

1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. O-O {[#]} Nf6 5. d3 d6 6. Bg5 h6 7.
Bh4 g5 8. Bg3 {[#] Black, not having castled, can advance the King's-side Pawns
} h5 9. h4 (9. Nxg5 h4 10. Nxf7 {[#]} hxg3 11. Nxd8 Bg4 12. Qd2 Nd4 13. Nc3
Nf3+ 14. gxf3 Bxf3 {'0-1 (14) Knorre,V-Chigorin,M St Petersburg 1874'} 15. hxg3
Rh1#) 9... Bg4 10. c3 Qd7 11. d4 exd4 12. e5 dxe5 13. Bxe5 Nxe5 14. Nxe5 Qf5
15. Nxg4 hxg4 16. Bd3 Qd5 17. b4 O-O-O 18. c4 Qc6 19. bxc5 Rxh4 20. f3 Rdh8 21.
fxg4 Qe8 22. Qe1 Qe3+ 23. Qxe3 dxe3 24. g3 Rh1+ 25. Kg2 R8h2+ 26. Kf3 Rxf1+ 27.
Bxf1 Rf2+ 28. Kxe3 Rxf1 29. a4 Kd7 30. Kd3 Nxg4 31. Kc3 Ne3 32. Ra2 Rxb1 33.
Rd2+ Kc6 34. Re2 Rc1+ 35. Kd2 Rc2+ 36. Kxe3 Rxe2+ 37. Kxe2 f5 38. Ke3 Kxc5 39.
Kd3 f4 {0-1 (39) Dubois,S-Steinitz,W London 1862} 0-1

And a great example of 'castle because you will or you must, but never because you can' (Pillsbury). Delayed castling allows an early Knight journey, a thrust by the h-pawn, and only later, long castling.

[Event "Wch04-Havana"]
[Site "Wch04-Havana"]
[Date "1892.??.??"]
[Round "4"]
[White "Steinitz, W.."]
[Black "Chigorin, M.."]
[Result "1-0"]
[ECO "C65"]
[PlyCount "55"]
[EventDate "1892.??.??"]

1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. d3 d6 5. c3 g6 6. Nbd2 Bg7 7. Nf1 O-O 8. Ba4
Nd7 9. Ne3 Nc5 10. Bc2 Ne6 11. h4 Ne7 12. h5 d5 13. hxg6 fxg6 14. exd5 Nxd5 15.
Nxd5 Qxd5 16. Bb3 Qc6 17. Qe2 Bd7 18. Be3 Kh8 19. O-O-O Rae8 20. Qf1 a5 21. d4
exd4 22. Nxd4 Bxd4 23. Rxd4 Nxd4 24. Rxh7+ Kxh7 25. Qh1+ Kg7 26. Bh6+ Kf6 27.
Qh4+ Ke5 28. Qxd4+ 1-0

Middlegame

Here's Bobby Fischer on Steinitz:

"He is the so-called father of the modern school of chess; before him, the King was considered a weak piece and players set out to attack the King directly. Steinitz claimed that the King was well able to take care of itself, and ought not to be attacked until one had some other positional advantage. He understood more about the use of squares than Morphy and contributed a great deal more to chess theory."

So, here is Steinitz' King looking after itself, and Steinitz showing a weak IQP is no basis for an attack on the King, Steinitz defending while being stubborn about concessions, then Steinitz accumulating other positional advantages.

[Event "Baden-Baden"]
[Site "Baden-Baden"]
[Date "1870.07.30"]
[Round "13"]
[White "Steinitz, William"]
[Black "Paulsen, Louis"]
[Result "1-0"]
[ECO "C25"]

1. e4 e5
2. Nc3 Nc6 3. f4 exf4 4. d4 Qh4+ 5. Ke2 d6 ({Another line goes} 5... d5 6. exd5
Bg4+ 7. Nf3 O-O-O 8. dxc6 Bc5 9. cxb7+ Kb8 10. Nb5 Nf6 11. c3 Rhe8+ 12. Kd3
Bf5+ 13. Kc4 $1 $18 Be6+ 14. Kxc5 a5 15. Nxc7 Qh5+ 16. Ne5 Nd7+ 17. Kb5 Qxd1
18. Nxd7+ Rxd7 $2 (18... Kxb7 19. Nc5+ Ka7 20. Nxe8 Rxe8 21. Kxa5) 19. Bxf4 $18
) 6. Nf3 Bg4 7. Bxf4 O-O-O 8. Ke3 Qh5 9. Be2 Qa5 10. a3 Bxf3 11. Kxf3 Qh5+ 12.
Ke3 Qh4 13. b4 g5 14. Bg3 Qh6 15. b5 Nce7 16. Rf1 Nf6 17. Kf2 Ng6 18. Kg1 Qg7
19. Qd2 h6 20. a4 Rg8 21. b6! axb6 22. Rxf6 Qxf6 23. Bg4+ Kb8 24. Nd5 Qg7 25. a5
f5 26. axb6 cxb6 27. Nxb6 Ne7 28. exf5 Qf7 29. f6 Nc6 30. c4 Na7 31. Qa2 Nb5
32. Nd5 Qxd5 33. cxd5 Nxd4 34. Qa7+ Kc7 35. Rc1+ Nc6 36. Rxc6# 1-0

A dramatic finish next, but it was all decided beforehand.

"If Zukertort has a plan in mind, he is a match for Steinitz, possibly even his peer. ... Every move of Zukertort's pointed towards a vigorous cooperation the pieces united to attack the King. This is the old Italian plan; Zukertort found it ready made, and in the tactics of execution he was a great master. Steinitz, however, discovered sound and successful plans over the board." LASKER

"It will be remembered that the capture of the c-Pawn was formerly held disadvantageous" - STEINITZ

"a system, imperfect though it may be, is preferable to move to move improvisation such as Zukertort resorted to in this game" - KONIG'

[Event "World-ch01 Steinitz-Zukertort +10-5=5"]
[Site "USA"]
[Date "1886.02.10"]
[Round "9"]
[White "Zukertort, J.."]
[Black "Steinitz, Wilhelm"]
[Result "0-1"]
[ECO "D26"]
[PlyCount "76"]
[EventDate "1886.01.11"]
[EventRounds "20"]
[EventCountry "USA"]

1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Nf3 dxc4 5. e3 c5 6. Bxc4 cxd4 7. exd4 Be7 8.
O-O O-O 9. Qe2 Nbd7 10. Bb3 Nb6 11. Bf4 Nbd5 12. Bg3 Qa5 13. Rac1 Bd7 14. Ne5
Rfd8 15. Qf3 Be8 16. Rfe1 Rac8 17. Bh4 Nxc3 18. bxc3 Qc7 19. Qd3 Nd5 20. Bxe7
Qxe7 21. Bxd5 Rxd5 22. c4 Rdd8 23. Re3 Qd6 24. Rd1 f6 25. Rh3 h6 26. Ng4 Qf4
27. Ne3 Ba4 28. Rf3 Qd6 29. Rd2 Bc6 30. Rg3 f5 31. Rg6 Be4 32. Qb3 Kh7 33. c5
Rxc5 34. Rxe6 Rc1+ 35. Nd1 Qf4 36. Qb2 Rb1 37. Qc3 Rc8 38. Rxe4 Qxe4 0-1

[Event "Havana"]
[Site "?"]
[Date "1888.??.??"]
[Round "?"]
[White "Steinitz, Wilhelm"]
[Black "Vasques"]
[Result "1-0"]
[ECO "C11"]
[Annotator "Euwe/Kramer"]
[PlyCount "63"]
[EventDate "1888.??.??"]

1. e4 e6 2. d4 d5 3. Nc3 Nf6 4. Bg5 dxe4 5. Nxe4 Be7 6.
Nxf6+ Bxf6 7. Bxf6 Qxf6 8. Nf3 O-O 9. c3 b6 10. Ne5 Nd7 11. Ng4 Qf4 12. Be2 Bb7
13. O-O f5 14. Ne3 Rf6 {Whether Steinitz overlooked something in the opening
stages, or whether he purposefully invited his opponent to attack, we shall
never know. Black's attack has assumed very dangerous proportions. Against
this must be set that the fact that White has not weakened his position in any
way. White must proceed with care; for instance, the attempt to change the
state of affairs by the seemingly energetic 15. Pd5 would be to his
disadvantage} 15. Re1 $1 {[%csl Re6] In full accord with the Steinitz theory
of defence. It avoids the weakening moves Pg3 or Ph3, and prepares to answer
15. ... Rh6 with Nf1. Meanwhile White begins to focus on the weak pawn on e6} (
15. d5 Rg6 16. dxe6 Qxe3 $3 {[%csl Rg2] Windmill attack} 17. fxe3 Rxg2+ 18. Kh1
Rxe2+ 19. Kg1 Rg2+ 20. Kh1 Rxb2+ 21. Kg1 Rg2+ 22. Kh1 Rxa2+ 23. Kg1 Rg2+ 24.
Kh1 Rg3+ 25. Rf3 Bxf3+ 26. Qxf3 Rxf3 $19) 15... Rh6 {[%csl Rh2]} 16. Nf1 {
[%cal Gf1h2]} Nf6 {[%cal Gf6g4]} 17. Bf3 $1 {An important decision. Steinitz
makes a concession, allowing his opponent to weaken his pawn structure.} Ne4 {
Black chooses to keep more pieces on the board to maintain attacking chances} (
17... Bxf3 18. Qxf3 Qxf3 19. gxf3 {[%csl Ge6,Yf2,Yf3] Black would no longer
havae a Kingside attack, and as compensation for the doubled and isolated
f-pawn - which is not easy to get at anyway - he would be able to operate
against Black's backward e-pawn}) (17... Ng4 18. h3 (18. Bxb7 Qxf2+ 19. Kh1
Nxh2 (19... Rxh2+ 20. Nxh2 Qh4 21. Kg1 (21. Qxg4 fxg4 22. Bxa8 g3 23. Kg1 $18))
20. Nxh2 Qh4 21. Kg1) 18... Bxf3 19. Qxf3 Qxf3 20. gxf3 Nf6 21. Rxe6) 18. Re3
Rf8 19. Qa4 a5 (19... Nd2 20. Bxb7 Nxf1 21. Kxf1 $18) 20. Rae1 {[%csl Re6]
building up latent pressure against the e-pawn} Rd8 {The Black attack has
already passed its peak} (20... Ba6 21. g3 Qg5 22. Bxe4 Bxf1 23. Bf3 $1 {
[%csl Re6] White wins the e-pawn}) 21. Ng3 {Now has come the time for White to
being the counterattack, and the first step is to remove Black's outpost on e4.
This however, means that White will have to weaken his King's position after
all, and this will automatically give Black renewed attacking prospects} Qh4
22. h3 {There defender comes under fire again, but his counterattack saves the
day} Nxf2 {This is the logical continuation of the attack} (22... f4 23. Rxe4
Bxe4 24. Rxe4 {[%cal Ge4h4]}) 23. Bxb7 Nxh3+ 24. gxh3 Rg6 $2 (24... f4 25. Rxe6
Rxe6 26. Rxe6 Qxg3+ 27. Bg2 f3 28. Re8+ Kf7 $1 $19) (24... f4 25. Bg2 {Bachmann
}) 25. Bg2 Rf8 (25... Rxg3 26. Rxg3 Qxg3 27. Rxe6 f4 28. Qc4 Kh8 29. Qe2 Rf8
30. Qf3) 26. Qc4 $1 {[%csl Re6] The tables are turned. Steinitz himself is now
making threats, and the look like they are more powerful than those of his
opponents} Kh8 27. Qxc7 f4 28. Rf1 Rxg3 {The only reasonable move as the f8
Rook is unprotected} 29. Qd6 $1 {[%csl Gf8] Preparing for the execution} Rd8
30. Rxf4 $3 {Black's position burst like a bubble} Rxg2+ 31. Kxg2 Qg5+ 32. Rg3
1-0 {Notes: Euwe/Kramer}

[Event "London"]
[Site "?"]
[Date "1876.??.??"]
[Round "?"]
[White "Steinitz, Wilhelm"]
[Black "Blackburne, JH."]
[Result "1-0"]
[ECO "C77"]
[Annotator "attack: very slow K-side attac"]
[PlyCount "55"]
[EventDate "1876.??.??"]
[EventType "tourn"]

1. e4 e5 2. Nf3 Nc6 3. Bb5 {Steinitz' first important
Lopez.} a6 4. Ba4 Nf6 5. d3 d6 6. c3 Be7 7. h3 O-O 8. Qe2 Ne8 9. g4 b5 10. Bc2
Bb7 11. Nbd2 Qd7 12. Nf1 {[#] The Steinitz trademark.} Nd8 13. Ne3 Ne6 14. Nf5
{[#] in a blocked position, loss of time cannot be taken advantage of so
easily. In this game, both sides re-position the knights.} g6 {[#] The moment
the dark squares are weakened, White snaps off their chief protector. but this
is a permanent weakening of the castled king's position} 15. Nxe7+ {removes
the defender of the dark squares} Qxe7 16. Be3 N8g7 17. O-O-O c5 {[#]White has
completed development and can play for direct attack} 18. d4 exd4 19. cxd4 c4
20. d5 Nc7 21. Qd2 a5 22. Bd4 f6 23. Qh6 b4 24. g5 {[#] [#] pounding at the
dark squares} f5 (24... Nge8 25. h4 Qg7 26. Qxg7+ Nxg7 (26... Kxg7 27. h5) 27.
gxf6 Nh5 28. Ng5 Nxf6 29. h5 Nxh5 (29... Kg7 30. hxg6 hxg6 31. Rh7+) 30. Rxh5
gxh5 31. Rg1 $18) 25. Bf6 Qf7 26. exf5 gxf5 {[#]} 27. g6 Qxg6 (27... Qxf6 28.
Qxh7#) 28. Bxg7 (28. Bxg7 {resigns} Qxh6+ (28... Qxg7 {heroic, but} 29. Rhg1
Rf7 30. Bxf5 $18) 29. Bxh6 Rf6 30. Rhg1+ Rg6 31. Bxf5 Kf7 32. Bxg6+ hxg6 33.
Ng5+ Kg8 34. Rge1 {[#] White's domination of the centre and King's-side, and
Black's failure to achieve counterplay on the Queen's-side (or centre), have
been the hallmarks of this game.}) 1-0

There's is more on Steinitz' theories on this website: here and here.

Endgame

Steinitz is not known for his endgame play, but try this one, a positional theme (two Bishops), evolving into an accompanying space advantage, and then a better King in the endgame. Some endgame niftiness: White's outside passed h-pawn is no help against such an active King.

[Event "London International Masters"]
[Site "London"]
[Date "1883.06.06"]
[Round "14.3"]
[White "Englisch, Berthold"]
[Black "Steinitz, Wilhelm"]
[Result "0-1"]
[ECO "C60"]
[Annotator "strategy: two bishops"]
[PlyCount "86"]
[EventDate "1883.04.26"]
[EventRounds "26"]
[EventCountry "ENG"]

1. e4 e5
2. Nf3 Nc6 3. Bb5 g6 4. d4 exd4 5. Nxd4 Bg7 6. Be3 Nf6 7. Nc3 O-O 8. O-O Ne7 9.
Qd2 d5 10. exd5 Nexd5 11. Nxd5 Qxd5 12. Be2 Ng4 13. Bxg4 Bxg4 14. Nb3 Qxd2 15.
Nxd2 Rad8 16. c3 Rfe8 17. Nb3 b6 18. h3 Be6 19. Rfd1 c5 {[#] Already denying
the Knight a central square. This is the recipe: the Knight more than any
other piece needs a secure advanced and/or central post. So Black keeps the
Knight from settling and works his way in using the long-range threats of the
Bishops.} 20. Bg5 f6 21. Bf4 Kf7 22. f3 g5 23. Rxd8 Rxd8 24. Be3 h6 25. Re1 f5
$1 {White doesn't want to allow f5-f4, but doesn't want to play f3-f4 either!}
26. f4 Bf6 27. g3 a5 28. Nc1 a4 29. a3 Bc4 30. Kf2 {[#] Black is now rather
better.} gxf4 31. Bxf4 Bg5 {Black chooses to go into a B vs N endgame. The
side with the two Bishops usually has a choice about how to change the
position, swapping one advantage for another.} 32. Bxg5 hxg5 33. Ke3 Kf6 34. h4
gxh4 35. gxh4 Re8+ 36. Kf2 Rxe1 37. Kxe1 Ke5 {Black's better King wins the
game.} 38. Ne2 Bxe2 39. Kxe2 {Both Bishops have disappeared, but Black is
winning.} Kf4 40. c4 Kg4 41. Ke3 f4+ {!} 42. Ke4 f3 43. Ke3 Kg3 {0-1
englisch-steinitz} 0-1

And an unbalanced endgame, arising from Steinitz' appetite for sacrificed pawns:

[Event "Hastings"]
[Site "Hastings, England"]
[Date "1895.08.21"]
[Round "13"]
[White "Mikhail Chigorin"]
[Black "Wilhelm Steinitz"]
[Result "0-1"]
[ECO "C52"]
[PlyCount "90"]
[EventDate "1895.08.05"]

1. e4 {Notes by Emanuel Lasker} e5 2.
Nf3 Nc6 3. Bc4 Bc5 4. b4 Bxb4 5. c3 Ba5 6. O-O d6 7. d4 exd4 8. cxd4 Nf6 9. e5
dxe5 10. Ba3 Be6 {So far everything is book. It has always been the opinion
that Black, although two Pawns ahead, will not be able to develop his Pieces,
as Castling King's side is prevented, and the King dare not occupy the centre
any length of time. Black seemed to believe that he may get his King safely
Castled to the Queen's side ; but this game does not corroborate such an
opinion, in spite of the success which attended that manoeuvre in this
instance.} 11. Bb5 {Tchigorin's favourite post for the Bishop in the Evans'
Gambit.} Qd5 12. Qa4 O-O-O 13. Bxc6 bxc6 14. Bc5 Bb6 15. Qa6+ {White
ultimately wins the exchange by this manoeuvre, but at an enormous expense. It
would have been better to leave things as they were, and to continue simply
with 15 Nxe5. If then 15...Nd7 16 Nxc6 must win the exchange in favourable
position; and if 15....Ne4 16 Bxb6 cxb6 17 Qxa7 will equalise the material
forces. with the position to White's advantage.} Kb8 16. Nxe5 Nd7 17. Nc3 Nxc5
18. Qe2 Qd6 19. dxc5 Qxc5 20. Na4 Qb5 21. Qxb5 cxb5 22. Nxb6 axb6 23. Nc6+ Kb7
24. Nxd8+ Rxd8 25. a3 c5 26. f3 Kc6 27. Rfd1 Ra8 {A Rook being very well
qualified to support advancing Pawns and to check the approach of the hostile
King, it is judicious play to avoid its exchange for the present.} 28. Kf2 Ra4
29. Ke3 h5 {Advancing these Pawns, which constitute the only weakness in
Black's camp, protects them against any possible attack of the Rooks or King.}
30. Kd2 b4 31. axb4 Rxb4 32. Rdb1 Rxb1 {Now it is just as well to simplify,
two united passed Pawns with the support of the King and Bishop being more
than a match for King and Rook. Black need only take care to leave the passed
Pawns as much as possible on the colour not dominated by the Bishop-i.e. on
black squares.} 33. Rxb1 b5 34. Ra1 b4 35. Kc2 Kd5 36. Rd1+ Kc4 37. Rd8 Bd5 38.
h4 Kd4 39. Rb8 Be6 40. Rb7 g6 41. Rb5 b3+ 42. Kb2 c4 43. Rb4 Kd3 44. Rb6 c3+
45. Kb1 Ke3 {threatening ...Bf5+ 45 g4 would be answered by
Kxf3. White is therefore perfectly helpless.} 0-1