user12957: How many openings does a typical (national/international level) tournament player usually know and play?
For instance: in a tournament, can I expect that a player will either open 1.d4 or 1.e4 consistently when they have white? And, if black plays (for instance) 1...e6, can I expect their move 2 to be always the same? How long of an opening line will they typically follow without changes (given the opportunity)?
Or maybe they have a single opening that they play 80% of the time, for instance, and then they change completely when they feel like doing something different?
I am not speaking about top-level matches such as a world champion match, where many matches between the same two players are played with a lot of preparation, and I can expect them to vary a bit more.

user2614: It's really less about the lines, and more about the ideas. To become strong, you have to know the typical middle game plans that arise from an opening. Knowing exact sequences of moves is less important, unless a certain sequence confers a concrete benefit (like saving a tempo or preventing counter play).
I wouldn't say that a player's "opening book" is important... it's their knowledge of positions and what plans and ideas to use in these positions that's important. Essentially these are patterns. If you know that a certain position is good for white or black, and what white or black should do in that position, that's a big advantage.

user-1: It depends.
Myself I play at a 1900 FIDE level and I have practically never studied openings!
Take as an example two players of my club rated 2250 FIDE:

Player A: theory enthusiastic: This player not only have a repertory close to an IM, but he went to a pub before each game to
study possible development of the games taking in consider what
openings his rival played.
Player B: tactics enthusiastic: This player do not study more theory and just played for fun at the club. He of course know some
basis because to reach that level you need to survive against every
+2000 the first ten moves, but he never prepared any game and uses to fall in second non book lines at the first moves.

Anyhow at a titled level everybody has a repertory. But also at this level there are some masters that know very deep lines of a huge repertory: more than fifty openings with more than ten lines on each one, to make a rude calculation, and others that just do not fall in ambushes, but as player B of my club can fall in non book lines to make think their opponents.

12957: <p>How many openings does a typical (national/international level) tournament player usually know and play?</p>

<p>For instance: in a tournament, can I expect that a player will either open <code>1.d4</code> or <code>1.e4</code> consistently when they have white? And, if black plays (for instance) <code>1...e6</code>, can I expect their move 2 to be always the same? How long of an opening line will they typically follow without changes (given the opportunity)?</p>

<p>Or maybe they have a single opening that they play 80% of the time, for instance, and then they change completely when they feel like doing something different?</p>

<p>I am not speaking about top-level matches such as a world champion match, where many matches between the same two players are played with a lot of preparation, and I can expect them to vary a bit more.</p>
 2614: <p>It's really less about the lines, and more about the ideas. To become strong, you have to know the typical middle game plans that arise from an opening. Knowing exact sequences of moves is less important, unless a certain sequence confers a concrete benefit (like saving a tempo or preventing counter play).</p>

<p>I wouldn't say that a player's "opening book" is important... it's their knowledge of positions and what plans and ideas to use in these positions that's important. Essentially these are patterns. If you know that a certain position is good for white or black, and what white or black should do in that position, that's a big advantage.</p>
 -1: <p>It depends.</p>

<p>Myself I play at a 1900 FIDE level and I have practically never studied openings!</p>

<p>Take as an example two players of my club rated 2250 FIDE:</p>

<ul>
<li><strong>Player A: theory enthusiastic</strong>: This player not only have a repertory close to an IM, but he went to a pub before each game to
study possible development of the games taking in consider what
openings his rival played.</li>
<li><strong>Player B: tactics enthusiastic</strong>: This player do not study more theory and just played for fun at the club. He of course know some
basis because to reach that level you need to survive against every
+2000 the first ten moves, but he never prepared any game and uses to fall in second non book lines at the first moves.</li>
</ul>

<p>Anyhow at a titled level everybody has a repertory. But also at this level there are some masters that know very deep lines of a huge repertory: more than fifty openings with more than ten lines on each one, to make a rude calculation, and others that just do not fall in ambushes, but as player B of my club can fall in non book lines to make think their opponents.</p>
