user25913: I was watching Irina Krush's great lecture on YouTube and, at 20:06, she asks "Which opening is this position from?" The answer is apparently the Sicilian, which made me wonder. How do you understand which middle game positions result from certain openings?
In the video example, I am guessing that the absence of Black's c pawn and the fight against the d5 square may have given it away. But can't that be the result of a different opening as well? Also, can we reverse engineer every middle game position to an opening, or do some openings result in obvious middle game positions?

user10380: Some positions have clues which at least rule out certain openings, even if they don't identify one specific opening. For example, if a pawn is still at home, then the opening wasn't one in which that pawn moves or gets taken.

user9684: The biggest clue is the pawn structure. Allure's first diagram hints that is was a Grunfeld due to the queenside pawns.  (Although since there are so many transpositions that the opening can change on a move's notice.)  The further the game proceeds, the harder it becomes to guess the opening.
As a teen, I played the Blackmar–Diemer Gambit, which had the trait of white missing the e and f pawns.  However, black could transpose into the French or the Caro-Kann, making a d pawn into an e pawn opening.

user15223: 
the absence of Black's C pawn and the fight against the D5 square

That's more or less it. This kind of position is most probably the result of a Sicilian. Could it be something else? It can, but the Sicilian is the most likely opening.

can we reverse engineer every middle game position to an opening

No. This position for example is a middlegame with no clear signposts as to what opening led to it.
[FEN "rn1r2k1/pp2qppp/8/8/3N2n1/1N2P3/P1Q2PPP/R4RK1 w - - 0 18"]

Here's another example. It should be decently clear which opening this game began with. If not, imagine Black playing ...f5 and White taking with exf5.
[FEN "r1bqk2r/pppn2bp/3p4/3Ppp2/2P5/2N1BP2/PP4PP/R2QKB1R w KQkq - 0 12"]


25913: <p>I was watching Irina Krush's great <a href="https://youtu.be/tsqhaNHqvY0?t=1204" rel="nofollow noreferrer">lecture</a> on YouTube and, at 20:06, she asks &quot;Which opening is this position from?&quot; The answer is apparently the Sicilian, which made me wonder. How do you understand which middle game positions result from certain openings?</p>
<p>In the video example, I am guessing that the absence of Black's c pawn and the fight against the d5 square may have given it away. But can't that be the result of a different opening as well? Also, can we reverse engineer <em>every</em> middle game position to an opening, or do some openings result in obvious middle game positions?</p>
 10380: <p>Some positions have clues which at least rule out certain openings, even if they don't identify one specific opening. For example, if a pawn is still at home, then the opening wasn't one in which that pawn moves or gets taken.</p>
 9684: <p>The biggest clue is the pawn structure. Allure's first diagram hints that is was a Grunfeld due to the queenside pawns.  (Although since there are so many transpositions that the opening can change on a move's notice.)  The further the game proceeds, the harder it becomes to guess the opening.</p>
<p>As a teen, I played the Blackmar–Diemer Gambit, which had the trait of white missing the e and f pawns.  However, black could transpose into the French or the Caro-Kann, making a d pawn into an e pawn opening.</p>
 15223: <blockquote>
<p>the absence of Black's C pawn and the fight against the D5 square</p>
</blockquote>
<p>That's more or less it. This kind of position is most probably the result of a Sicilian. Could it be something else? It can, but the Sicilian is the most likely opening.</p>
<blockquote>
<p>can we reverse engineer every middle game position to an opening</p>
</blockquote>
<p>No. This position for example is a middlegame with no clear signposts as to what opening led to it.</p>
<pre><code>[FEN &quot;rn1r2k1/pp2qppp/8/8/3N2n1/1N2P3/P1Q2PPP/R4RK1 w - - 0 18&quot;]
</code></pre>
<p>Here's another example. It should be decently clear which opening this game began with. If not, imagine Black playing ...f5 and White taking with exf5.</p>
<pre><code>[FEN &quot;r1bqk2r/pppn2bp/3p4/3Ppp2/2P5/2N1BP2/PP4PP/R2QKB1R w KQkq - 0 12&quot;]
</code></pre>
