May I ask how we determine that a game has transposed from the opening to the middlegame?

We don't, well is only a theory, once the king has castled, most of your pieces are active ready to start exchanging/attacking maybe that is when is in the middlegame stage.

There's no perfect answer to this.  Most openings are based on solid theory about 10 moves long (both white/black), but there are quite a few openings that have been thoroughly explored up to 30 or 40 moves in.

Wikipedia, the ever-knowing source for anything you want to know, cites the following criteria for what you want to accomplish during the opening phase:

1. Development: One of the main aims of the opening is to mobilize the pieces on useful squares where they will have impact on the game.
2. Control of the center .
3. King safety: usually via castling.
4. Prevention of pawn weakness: striving to avoid the creation of pawn weaknesses such as isolated, doubled and backward pawns.
5. Piece coordination: As each player mobilizes his or her pieces, each attempts to assure that they are working harmoniously towards the control of key squares.
6. Create positions in which the player is more comfortable than the opponent: i.e., Transposition.

As one or more of these criteria are met, the game becomes more developed and moves further away from the strict opening sense.

One of the rules-of-thumb that I like to use is whether or not any rook has moved (in a non-castling move) or been taken.  This is usually a clear sign that the game is no longer in an opening stage.

But what if the Rook hasn't moved...or been taken during an ENTIRE game?!

After 14 moves.

rich, is that from your experience or something? Or did you find it in books?

he is just b.s. you turn

oh.......really?

So do you know the answer? If you don't, feel free to post your opinion...

i heard it on a chess video.

which one and who said that ?

But does anyone have a more accurate answer?

jrobichess

Middle-game, the phase of the game generally beginning between the 11th and 20th moves that follow the opening. However, the change from one to the other is often imperceptible, with no clear demarcation.

source: The Oxford Companion to Chess, David hooper and Kenneth Whyld

kco, good answer.

thank you

This is why we should not study chess as three seperate parts, it is one whole game and should not be arbitrarily classified as such.

I kind of prefer to use 'the early part of the game' instead of 'opening'.

 At move 30 you are certainly no longer in the opening. Yes, some opening books go to move 30 and beyond but are certainly in middlegame territory by that point...  this is confusing for some. I would say that more than 90% of games are in middlegame territory by move 15 or so. As a rule of thumb when you are looking where to place your rooks then you are in the middlegame phase, more often than not. I think kco's post is quite accurate and I agree with the definition there. I would say an opening ends somewhere between move 10 to 15 and the middlegame begins.

Is the opening also referred to ad the beginning, then?

The middlegame is entered when the goal of one or both players changes from "I need to develop my pieces and connect my Rooks" to "I need to come up with a plan of action now." Once someone has connected the Rooks, you're probably in the middlegame. And if you're playing with no plan after you connect the Rooks, ah, that really doesn't work so well once you get into the 1600s.

 Sure... After 15 moves.

lol, I believe the Middlegame starts after you develop all the Pieces and are striving to gain the advantage strategically or positionally.