What is Middle game?

 The 'Middle Game' is that stage of the chess play when the opening moves are indulged in the thick battle of wits. The Middle game is still the most engrossing part of chess play. Opening stage, Middle game stage, and Endgame.. But the Middle game stage is the best part I want in chess game.. I make an example for how we know if it will be the middle game.

  Sacrificing the pieces for an mobility advantage.

  So the white wins by sacrificing the queen.

 DC-poc- I'm not showing great puzzles (or whatever).. But How the Middle game stage enter the chess game..

  nyLsel :D

  Thanks for that encouragement words.. If you have better puzzles you can post it here. I will appreciate your cooperation..

 nyLsel

The Middle Game is the part that fills in the space between the last move of the Opening and the first move of the Endgame.

I hope that helps.

 The middle game starts when the preparation is over ( the opening )

 The middle game forms a complete whole based on one single theme. But often, on the contrary, it is made up of a strategic phase. B.K Chaturvedi

for me middlegame starts after 8 or ten moves after opening,whatever advantages ACQUIRED IN THE OPENING IS TREATED MORE SERIOUSLY IN THE MIDDLEGAME.

If a chess statistician were to try and satisfy his curiousity over which stage of the game proved decisive in the majority of cases, he would certainly come to the conclusion that it is the middlegame that provides the most decisive stage.  -  Alexander Kotov

The middlegame I repeat is chess itself, chess with all its possibilities, its attacks, defences, sacrifices, etc.  -  Eugene Znosko-Borovsky

Books on the openings abound; nor are works on the end game wanting; but those on the middle game can be counted on the fingers of one hand.  -  Harry Golombek

Your only task in the opening is to reach a playable middlegame.  - Lajos Portisch

Before the endgame the gods have placed the middlegame.  - Siegbert Tarrasch

the-essence-of-chess

Chess is a 32 piece endgame. - Garry Kasparov

Nice    

I think Capablanca said it's a process of clarification. I like that notion. In the middle game, you are removing everything (pawns, pieces, and locations) that are not the end game you want. Assuming no mates occur, of course.

The opening ends and the middle game begins almost always at the move known as the 'break'.  The 'break' is usually a pawn move that is the first move of the middle game plan of attack.  It usually occurs as early as move 10 and as late as move 30. I will explain what a 'break' move is a little later on in this post.  The plan of attack and the 'break' move chosen is dictated by (2) two factors: 1. The opening selected, and 2. How Black chooses to defend.  Almost all openings result in (6) characteristic pawn structures.  A good overview and how to handle them as Black or White, of those 6 characteristic pawn structures, is covered in, "Pawn Power In Chess", by Hans Kmoch.  In his book Mr. Kmoch writes about what he labels as 'levers'.  'Levers' are created when two pawns are diagonally adjacent to each other, and each can excute a capture of the other (depending on whose move it is).  [I wish I knew how to post interactive diagrams on this post.  It would help alot with this explanation.]  A 'break' move is usually a pawn move that creates a 'lever' with one of your opponent's pawn.

I wrote earlier that the 'break' move selected is dictated by two factors.  In every opening there are thematic plan(s) of attack.  By studying opening books you learn about those plans of attack in the opening(s) you have chosen to study.  How your opponent chooses to defend determines what thematic plan of attack you will use and also when and where on your opponents pawn structure you will execute your 'break' move.  

 I hope this helps and hopefully answers your question.

I also think that pawn breaks can be the border between opening and middlegame. The middlegame is execution of a plan, and as long as the pawn structure is not being changed, players are rather preparing that plan and repositioning for it.

I'll check Hans Kmoch's book. I've read another book on similar thematics - "Pawn Structure Chess" by Andrew Soltis, where he also analyzes different pawn structures and shows appropriate game plans for them.