user915: 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.

user70: 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.

user1161: 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. 

user17: 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.

915: <p>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</p>
<blockquote>
<p>All pieces developed</p>
<p>King castled</p>
<p>Rooks connected</p>
</blockquote>
<p>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.</p>
<p>I'm pretty sure there is no hard and fast rule such as</p>
<blockquote>
<p><em>By move 10 all players should be thinking middle game, regardless of position</em></p>
</blockquote>
<p>As all games are different (sometimes one side plays aggressive and there may be no time to castle).</p>
<p>I see a related question on <a href="https://chess.stackexchange.com/questions/106/when-does-the-middlegame-end-and-the-endgame-begin">the line between middle game and endgame</a>, but not this one.</p>
 70: <p>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.</p>
 1161: <p>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. </p>
 17: <p>The opening is for "deploying." The middle game is for "fighting." So the middle game begins when the fighting begins in earnest.</p>

<p>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.</p>

<p>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.</p>
