user31998: According to Irving Chernev in his summary of Queen's pawn opening on page 100 in Logical Chess there is one important step black should do in any Queen's Pawn opening ...c5. How true is this today? 

user9684: (I didn't read the excerpt, so I might repeat his summary.)
The purposes of the c5 break is to destroy white's center, free your pieces, and to ease the game.  This can be accomplished by the e5 break, but most lines (back then) had b6 played to develop the bishop, so c5 was easier to force.
There are many top level games where the restricted black queen's bishop is the cause of a lost game.  There are some examples of black playing a tough defensive game without any center action, but these are difficult.
The axiom is still true and more useful than many rules-of-thumb. such as passed pawns must be pushed.

user21110: How true is this today? As true as it was then. Extremely true and important.
The reason is simple: it is a principle of middle-game theory that both sides should strive to achieve a central pawn break. In Queen Pawn games, white has a central pawn in d4, which leaves two pawn breaks for black: e5 or c5.
e5 is generally difficult to achieve because of several reasons. Firstly, white tends to have great control of such square with a knight on f3 or bishop on f4 (or g3 in some lines). On the contrary, white's pieces do not tend to have much control of c5 on this openings.
In simpler words, you really want a pawn break and while e5 is difficult c5 is easy.
As a plus, it is better to trade white's d4 pawn for your c-pawn rather than your e-pawn, the reason being that it is generally preferable to trade a side-pawn for a central-pawn.

31998: <p>According to Irving Chernev in his summary of Queen's pawn opening on page 100 in <em>Logical Chess</em> there is one important step black should do in any Queen's Pawn opening ...c5. How true is this today? <a href="https://i.stack.imgur.com/yKiaI.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yKiaI.jpg" alt="enter image description here" /></a></p>
 9684: <p>(I didn't read the excerpt, so I might repeat his summary.)</p>
<p>The purposes of the c5 break is to destroy white's center, free your pieces, and to ease the game.  This can be accomplished by the e5 break, but most lines (back then) had b6 played to develop the bishop, so c5 was easier to force.</p>
<p>There are many top level games where the restricted black queen's bishop is the cause of a lost game.  There are some examples of black playing a tough defensive game without any center action, but these are difficult.</p>
<p>The axiom is still true and more useful than many rules-of-thumb. such as passed pawns must be pushed.</p>
 21110: <p><em>How true is this today?</em> As true as it was then. Extremely true and important.</p>
<p>The reason is simple: it is a principle of middle-game theory that both sides should strive to achieve a central pawn break. In Queen Pawn games, white has a central pawn in d4, which leaves two pawn breaks for black: e5 or c5.</p>
<p>e5 is generally difficult to achieve because of several reasons. Firstly, white tends to have great control of such square with a knight on f3 or bishop on f4 (or g3 in some lines). On the contrary, white's pieces do not tend to have much control of c5 on this openings.</p>
<p>In simpler words, you really want a pawn break and while e5 is difficult c5 is easy.</p>
<p>As a plus, it is better to trade white's d4 pawn for your c-pawn rather than your e-pawn, the reason being that it is generally preferable to trade a side-pawn for a central-pawn.</p>
