user28131: When searching the web about how people learn about chess openings, I see many questions regarding the depth of the opening knowledge one has (with answers such as "I know some lines 20 moves deep") but a line is just a sequence of moves not a tree of possible moves.
This leads me to the following question : what is the best tradeoff between learning some lines really deep vs learning many variations but not as deep? How does one decide what lines should be learnt deeper?

user13012: Chess should be a game of understanding and not of memorization. In large part this is because understanding is a whole lot more fun than memorizing. Memorization has become prominent because it has become seen as necessary at the very top levels, and perhaps it is, but that should not be of concern if you are content with less. And not all that much less; you can be a serious contender for most state championships
without keeping abreast of the latest opening theory. It has been said, very properly, that the ONLY purpose of knowing the openings is to enter the middlegame with a playable position.
Brian Towers suggests "cutting off" opening theory when the position has become "calm", but although this is good advice, the position does not always take the same number of moves to do this. Some lines remain unclear after thirty moves, but if you are intimidated by this, do not enter those lines. There are many players, even at world championship level, whose aim, to make an analogy with tennis, is merely to keep the ball in play without aiming for every shot to be a winner (just a little bit tricky). There is no rule saying you have to play the most theoretical lines.
I am reminded of something once said by Bent Larsen, in recommendation of the Nimzoindian defence, that to play for advantage with Black you must first make some concession. And that in his view, giving up a Bishop for a Knight was not such a serious concession to make. This is opening theory at the level of understanding. Read everything you can about it, and enjoy the game.

user4418: 
what is the best tradeoff between learning some lines really deep vs learning many variations but not as deep?

There are several cut-offs you can use:

Cut short the depth in a particular line when it becomes "calm". So, both sides have developed and there are no immediate tricky tactics.
As a first cut on breadth, make sure you include critical lines which have tactics which you need to know to survive. Do this even if they are relatively unpopular. You don't want your first game in that variation to be a quick loss.
For less critical lines cut short the breadth according to popularity, unless you particularly like an unpopular line. In general you get the most benefit by studying the most popular lines first because they are the ones you will face the most often. Use a large database to identify these and make a cut-off point in popularity below which you are not going to spend much time. Only study an unpopular line after you have faced it as part of your post-game analysis.
Another useful breadth cut-off if you don't have a great deal of time to spend is to do so according to how much your choice limits your opponent's choice. Examples would be: As an e4 player with white against the French only study the Exchange Variation (even though it is less popular) because it allows you to limit your opponent's choice of moves and so drastically reduce the amount of learning you have to do. Same argument for playing the Exchange Variation against the Slav.


28131: <p>When searching the web about how people learn about chess openings, I see many questions regarding the depth of the opening knowledge one has (with answers such as &quot;I know some lines 20 moves deep&quot;) but a <em>line</em> is just a sequence of moves not a tree of possible moves.<br />
This leads me to the following question : what is the best tradeoff between learning some lines really deep vs learning many variations but not as deep? How does one decide what lines should be learnt deeper?</p>
 13012: <p>Chess should be a game of understanding and not of memorization. In large part this is because understanding is a whole lot more fun than memorizing. Memorization has become prominent because it has become seen as necessary at the very top levels, and perhaps it is, but that should not be of concern if you are content with less. And not all that much less; you can be a serious contender for most state championships
without keeping abreast of the latest opening theory. It has been said, very properly, that the ONLY purpose of knowing the openings is to enter the middlegame with a playable position.</p>
<p>Brian Towers suggests &quot;cutting off&quot; opening theory when the position has become &quot;calm&quot;, but although this is good advice, the position does not always take the same number of moves to do this. Some lines remain unclear after thirty moves, but if you are intimidated by this, do not enter those lines. There are many players, even at world championship level, whose aim, to make an analogy with tennis, is merely to keep the ball in play without aiming for every shot to be a winner (just a little bit tricky). There is no rule saying you have to play the most theoretical lines.</p>
<p>I am reminded of something once said by Bent Larsen, in recommendation of the Nimzoindian defence, that to play for advantage with Black you must first make some concession. And that in his view, giving up a Bishop for a Knight was not such a serious concession to make. This is opening theory at the level of understanding. Read everything you can about it, and enjoy the game.</p>
 4418: <blockquote>
<p>what is the best tradeoff between learning some lines really deep vs learning many variations but not as deep?</p>
</blockquote>
<p>There are several cut-offs you can use:</p>
<ol>
<li>Cut short the depth in a particular line when it becomes &quot;calm&quot;. So, both sides have developed and there are no immediate tricky tactics.</li>
<li>As a first cut on breadth, make sure you include critical lines which have tactics which you need to know to survive. Do this even if they are relatively unpopular. You don't want your first game in that variation to be a quick loss.</li>
<li>For less critical lines cut short the breadth according to popularity, unless you particularly like an unpopular line. In general you get the most benefit by studying the most popular lines first because they are the ones you will face the most often. Use a large database to identify these and make a cut-off point in popularity below which you are not going to spend much time. Only study an unpopular line after you have faced it as part of your post-game analysis.</li>
<li>Another useful breadth cut-off if you don't have a great deal of time to spend is to do so according to how much your choice limits your opponent's choice. Examples would be: As an e4 player with white against the French only study the Exchange Variation (even though it is less popular) because it allows <em>you</em> to limit your opponent's choice of moves and so drastically reduce the amount of learning you have to do. Same argument for playing the Exchange Variation against the Slav.</li>
</ol>
