user13565: I'm looking for a site that provides well-annotated openings, giving at least the first 15-20 moves of major openings and allowing me to explore their main variations and lines (by clicking on the relevant moves on an online chessboard).
For example, if I am learning Queen's Gambit Declined, I'd expect it to display something along the lines of this (more detailed and expertly annotated, of course, and with a diagram)

1. d4 A popular  opening move, which gives White a solid position. 1. ... d5 Black's reply is quite common, and preserves the symmetry of the situation. 2. c4 This is the Queen's gambit - if accepted, it can provide White great momentum and development. 2. ...e6 Black turns down the gambit  and prefers to keep the position interesting. 2. ...dxc4 is not really got for Black as White can get the pawn back soon by [sequence of moves] followed by a bishop move.

And if I play 2. ...c6, I'd expect something like this (but again more detailed and with a diagram to help.)

2. ...c6 This changes the opening to the Slav defense.

My question
Is there any site online that provides detailed, annotated openings at least 15-20 moves deep and allows me to explore variations and lines (also
annotated)?
Note
The site needn't focus on other parts of the game. I'm focusing on the opening for now.

user12523: Wikibook Chess Opening Theory has similar aims but I don't think it quite meets all of your requirements. Still, it's the website closest to meeting them that I can think of, so I hope it helps. Being a wiki, it's still evolving and of course one can always wonder about the expertise of the authors.

13565: <p>I'm looking for a site that provides <strong>well-annotated openings</strong>, giving at least the first 15-20 moves of major openings and allowing me to explore their main variations and lines (by clicking on the relevant moves on an online chessboard).</p>
<p>For example, if I am learning Queen's Gambit Declined, I'd expect it to display something along the lines of this (more detailed and expertly annotated, of course, and with a diagram)</p>
<blockquote>
<p><strong>1. d4</strong> A popular  opening move, which gives White a solid position. <strong>1. ... d5</strong> Black's reply is quite common, and preserves the symmetry of the situation. <strong>2. c4</strong> This is the Queen's gambit - if accepted, it can provide White great momentum and development. <strong>2. ...e6</strong> Black turns down the gambit  and prefers to keep the position interesting. <em>2. ...dxc4</em> is not really got for Black as White can get the pawn back soon by [sequence of moves] followed by a bishop move.</p>
</blockquote>
<p>And if I play <strong>2. ...c6</strong>, I'd expect something like this (but again more detailed and with a diagram to help.)</p>
<blockquote>
<p><strong>2. ...c6</strong> This changes the opening to the Slav defense.</p>
</blockquote>
<h2>My question</h2>
<p>Is there any site online that provides <strong>detailed, annotated openings at least 15-20 moves deep</strong> and allows me to explore <strong>variations and lines (also
annotated)</strong>?</p>
<h2>Note</h2>
<p>The site needn't focus on other parts of the game. I'm focusing on the opening for now.</p>
 12523: <p><a href="https://en.wikibooks.org/wiki/Chess_Opening_Theory" rel="nofollow noreferrer">Wikibook Chess Opening Theory</a> has similar aims but I don't think it quite meets all of your requirements. Still, it's the website closest to meeting them that I can think of, so I hope it helps. Being a wiki, it's still evolving and of course one can always wonder about the expertise of the authors.</p>
