At what point does the opening end, and middle game begin?

Question

Just curious if there's a set of guidelines to determine when a player can call the opening pretty much done, and move into the middle game. I know some rules of the opening that would apply are

All pieces developed

King castled

Rooks connected

And by those definitions, it's easy to see that white may begin a middle game strategy while black is still in the opening phase, or vice versa.

I'm pretty sure there is no hard and fast rule such as

By move 10 all players should be thinking middle game, regardless of position

As all games are different (sometimes one side plays aggressive and there may be no time to castle).

I see a related question on the line between middle game and endgame, but not this one.

Answer 1

The opening is for "deploying." The middle game is for "fighting." So the middle game begins when the fighting begins in earnest.

Think of two armies marching to a battlefield. The opening is when they "leave camp." The middle game is when they arrive on the battlefield and get down to fighting.

If there is serious "fighting" a middle game can begin as early as the seventh or eighth move. On the other hand, the middle game can begin as late as the fifteenth move or so if both sides opt for a "slow" development.

Answer 2

I don't remember the exact wording or even the source but I have seen a definition of where the opening ends and where the middle game begins as the point where the players leave prepared opening lines and start to innovate. This would vary from player to player, new players may leave their understanding of prepared opening lines in one or two moves, while grand masters may understand the opening lines, pre-prepared understanding of positional specifics, into 15 moves or more.

Answer 3

Roughly speaking, when most knights and bishops are off the back row and kings are casted, the middlegame has begun. There's no hard and fast rule, however.

Answer 4

The dividing lines between the various stages of the game are somewhat fluid, but a good rule of thumb is that the opening ends when all your pieces are developed to their best squares and you have castled. Of course you are considering middle games strategies while you are making some of these moves, which accounts for the ambiguity. The same is true of the demarcation between middle and endgames, although that's often classified as when the queens leave the board. I guess the bottom line is to view the entire game as an integrated unit with subtle transitional stages along the way.