There are different ways to study the middlegame. On the one hand, the opening repertoire can be used as a starting point for examining the pawn structures of the most important variations. The aim is to fathom the peculiarities of the respective structure, i.e. the value distribution and the resulting plans. Of course, I’ll go into that during my opening training sessions. The advantage of this method is the great relevance for your own practice.

An alternative procedure is to approach the middlegame on the level of the strategic concepts. This is more abstract, but leads to a comprehensive improvement in chess understanding. I recommend taking both paths, as they complement each other well. For this general middlegame training I have a collection of thematically ordered sample games. Here is an overview of the topics:

Activity has several manifestations: lead in development, initiative, attack or just space-grabbing moves. Rule of thumb: When in doubt, play the more active move.

Attacks must be well prepared. If they are repelled, this is often associated with a loss of time or material or the presence of permanent weaknesses. It is often worth sacrificing material for an attack, since attacking is easier than defending. The defender has to calculate more variations and the weight of his mistakes is higher than that of the attacker.

Bad Pieces
Bad pieces can be a consequence of opponent’s restrictive measures or they can result naturally from the choice of the opening (e.g. the bad French bishop). They usually form the basis of an entire strategy (exploiting the bad piece). This can be the shifting of the game to the wing, where the piece cannot work, or the liquidation into an ending “good versus bad minor piece”.

More often than we think, we fall victim to perceptual deception.

Barriers of Perception
The most beautiful moves are those that are camouflaged by barriers of perception. They are only visible for very experienced players, who’s thinking is flexible enough to break the rules. Here are a few examples of rule-breaking:

  • A piece enters a controlled square (controlled by a pawn would be the highest level of difficulty).
  • A mutilation of the pawn structure is tolerated.
  • A piece is moved to the edge.
  • A piece retreats to the first rank.
  • The queen makes a small move.
  • A piece moves towards a pieces of the same kind without capturing it.
  • The king behaves unusually, e.g. by staying in the center.
  • A single piece consumes many tempi in succession.

Center Formations
The study of typical pawn structures is at the top of the list of priorities in middlegame training. On the one hand, there is the generic distinction between “open center”, “closed center”, “static center”, “dynamic center” and “mobile center” (according to Kotov). In addition, one can be more specific and speak of the Karlsbad-structure, the isolated pawn center, the hanging pawns, the Hedgehog-structure, the Caro-Kann-structure or the Scheveningen-center, to name just a few.

Passive defense is often a bleak endeavor. So, while you’re still able to, keep an eye out for opportunities to generate counterplay. On the other hand, if you enjoy a large static superiority, it is of the highest priority to prevent counterplay. The realization of the static advantage does not run away.

Many players do not like to defend. However, the art of the defense should still be learned, since one can get into a defensive situation against one’s own will. In many openings, defense is part of the strategy, namely when you grab a static advantage (the better pawn structure), as in the Sicilian. Those who are not prepared to defend rarely get such winning ideas with black.
Bishops on the long diagonals can strongly influence the overall position. Just think of the Catalan bishop, which can provide a long-lasting white initiative in a rather boring symmetrical pawn structure.
Effectiveness is the real yardstick for judging pieces, as opposed to visual appearance, which can be deceptive. So a bad bishop, reliably defending a pawn, can be an important part for the whole position. The value of the pieces depends solely on the extent to which they perform work and contribute to the achievement of strategic goals, both offensive and defensive.
The skill of making the right exchanges is of great importance. After all, it is the simplest form of gaining material, having the relative value of the pieces in mind.

Since chess is an information war, flexibility is a valuable asset. Keeping my options open means two things: 1) I can collect more information before committing to a definite plan. 2) My opponent has to reckon with several plans and therefore play moves that cover all possibilities. This compromise naturally leads to a dilution of his game.

Global Thinking
We tend to focus on the central areas of the board. Strong players also include the outskirts and see how activities there can impact other sections of the board.

Hypermodern Strategy
In the 1920s, the hypermodern school supplemented the classical school with further methods of fighting for center control. In addition to occupying the center with pawns, the center was now also readily controlled by neighboring pawns, often supported by a fianchettoed bishop. The central squares themselves sometimes served as welcome bases for knights. As White, this strategy is best implemented with 1.c4 or 1.Nf3, and as a Black, with the Nimzo-Indian/Queen’s Indian against 1.d4 and the Sicilian against 1.e4.

Ideal Squares
In the middlegame it is mostly about piece improvement. Here you should try to imagine which would be the optimal squares for the pieces. So you don’t shimmy from move to move, but think from the end. Maneuvers that can take many moves are easily discovered this way.

Playing with initiative means creating threats most of the time. In this way, the opponent is forced to react, which severely limits his options. Additionally, the constant pressure makes him prone to commit mistakes.

King Safety
King safety is highly valued by strong players. A notoriously vulnerable king, caused by a loose pawn structure (moves with the f-pawn should be carefully considered), binds pieces to the protection of the king, which of course limits their radius of action.

Material Imbalances
In addition to sacrifices, this also includes roughly balanced but asymmetrical material ratios: queen against 2 rooks, bishop pair, bishop against knight, rook and 2 pawns against two minor pieces, etc.

Minority Attack
Usually you attack where you have a majority of pawns. But there is also the possibility of a minority attack with the aim of provoking pawn weaknesses in the opponent’s camp. In my 1.d4 repertoire this occurs in the Queen’s Gambit Exchange Variation.

Open Lines
Open lines for the rooks are important to invade the enemy position or to attack the king. Sometimes you have to actively create them, in many cases also by sacrificing pawns. This also applies to the clearing of diagonals for the bishops or attractive squares for a knight.

95% of the time it is knights, who benefit from outposts. Here, one can distinguish between real and quasi-outposts. In the case of a real outpost, the outpost cannot be attacked by an opposing pawn. In the case of a quasi-outpost, this can be done, but only under difficult conditions (the pawn is blocked or the move of the pawn would weaken the neighboring pawn).

Passed Pawns
Advanced passed pawns are a material phenomenon as converting them into a queen would mean a huge gain. The same applies, if a minor piece has to sacrifice itself for the passed pawn. But even if the opponent’s pieces managed to block the passed pawn on the seventh or eighth rank, it would lead to a lethal paralysis. In the endgame, an outside passed pawn causes forces to be withdrawn from the center, allowing the opponent to potentially perform an invasion there.

Pawn Breaks
The pawn-break potential is an important element of the position. Each lever is a potential plan. If a position suffers from absence of breaking potential, this can result in passivity. The greatest lever talent is certainly the hedgehog. Here, all seven pawns can actually be used for this. The most important break is …b6-b5.

Pawn Majorities
Positions with different pawn majorities are usually more challenging than those with simpler distributions. A positional advantage often results from the fact that one’s own majority is more mobile than that of the opponent. Majorities of pawns can be used to gain space, to attack or to create passed pawns.
Pawn Mobility
Pawn mobility is the generic term for pawn majorities, pawn-break potential and the presence of passed pawns.
Pawn Structure
The pawn structure is a prominent element of the position, as it determines the texture of the playing field and hence the value of the pieces and the appropriate plans. It is a generic term for weak pawns, weak squares, pawn mobility, spatial conditions (spatial advantage) and typical center formations. 
Penetration is a key offensive objective. The classic example is occupying the 7th rank or even doubling rooks there.
Piece Activity
Among the four elementary value areas “material”, “piece activity”, “pawn structure” and “king safety”, the piece activity takes on a special role, since it can best be regarded as a value in itself. All other areas get their meaning by reference to piece activity. An intact pawn structure is of limited value, if it offers the pieces little room to maneuver. One must not forget that the pieces are the heavy guns and therefore the pawns must put themselves at their service.

Positional Sacrifice
A positional sacrifice is a long-term sacrifice in exchange for other values in the areas of “piece activity”, “king safety” or “pawn structure”. Basically, such sacrifices are the domain of stronger players, since assessing the compensation is often not a trivial matter. But I also encourage normal club players to study this topic, as it is a good way to broaden their chess horizons. There are plenty of nobrainer sacrifices, who tend to be ignored, but are actually within reach of many chess enthusiasts.

Prophylaxis can be understood to mean several things: 1) protecting important points and reducing weaknesses, 2) preventing concrete enemy plans, and 3) defusing concrete enemy plans. The latter point can also be connected to a trap, so that the opponent runs to his undoing, if he insist in executing his plan.

Provoking weaknesses, primarily by forceful play, is one of the main goals of seasoned players. After all, it is weakness you are supposed to attack, not strength.

Restriction is Karpov’s favorite discipline, who sees chess primarily as a game of constriction. It is the part of piece activity, which doesn’t deal with increasing the scope of our pieces, but with the reduction of the opponent’s piece power. This is, of course, worth just as much. It is usually accomplished through skillful pawn play.

If you don’t have a way to force weaknesses on your opponent, you can still use seduction to trick him into making an unforced error. In the Hedgehog, for example, Black often maneuvers his pieces in the hope that White will move too many pawns onto the fourth rank. For this purpose, for example, the knights dance around on e5 and c5 or the bishop may provocatively position itself on g5.
A simplification is conducted by players, who are aiming for a draw or enjoy a static advantage, be it material or positional. In theses cases one tries to eliminate the position of its (dynamic) impurities until finally the pure static advantage remains as a distillate.

Space Advantage
Space advantage is a function of the pawn structure and derives its value from its relationship to piece activity and king safety. It is desirable when it increases your piece activity and reduces your opponent’s, which is the case most of the time. When pieces are exchanged, the effect of the space advantage tends to decrease, so that it can assume a neutral value. But it also happens that its weakening character moves into the foreground. We mustn’t forget, expansion comes with obligation! If at some point you lack the necessary pieces to defend the conquered space, the enemy can break through the front line.

Static vs. dynamic Advantage
The most interesting duels sometimes occur in positions with an asymmetrical value distribution, in which one side has dynamic advantages and the other has static ones. From this constellation, the plans emerge all by themselves. The player with the static advantage knows he owns the future, should he only happen to see it. His task is to consolidate the position, to ward off short-term dangers and, if necessary, to bring about simplifications. The player with the dynamic advantage is aware that he needs to seize the moment before his initiative fizzles out. This does not have to mean an uncompromising attack, but can also be done with a value transformation from dynamic to static.

Transformation of Advantages
A chess game can be understood as a marketplace where advantages (values) are shifted, exchanged and transformed. Normally, the flow of values ​​runs from dynamic to static as desired. Dynamic advantages (lead in development, initiative, attack) are often exchanged for a better pawn structure or for material advantages (a pawn, the two bishops, etc.). Static advantages are of course more tangible and cannot be lost that easily. But sometimes it works the other way around, e.g. sacrificing a pawn for a strong attack. But here, you also have something solid in mind, such as mate or recuperating the material with strong interest. Of great practical importance is the transformation of a material advantage into a positional advantage, in order to make the position easier to play. The opponent had previously only insufficient compensation in the form of cheating chances, but now the game has become a one way road.

Weak Pawns
Weak pawns are a key issue, as the poorly mobile pawns make excellent targets. Even if this does not result in a nominal loss of material, it causes binding of the defending pieces. Actually, this corresponds to a real material loss, since the affected pieces have to be devalued.
Weak Squares
Weak squares can lead to the enemy gaining outposts or even being able to penetrate the position. Since every pawn move weakens squares, certain weakenings cannot be avoided. However, one has to carefully weigh up how relevant they are.
I am willing to expand this glossary, if there is a concrete demand for a topic.