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+<head>
+<title>UK TeX FAQ -- question label dolldoll</title>
+</head><body>
+<h3>Why use <code>\</code><code>[</code> ...<code>\</code><code>]</code> in place of <code>$$</code> ...<code>$$</code>?</h3>
+<p>LaTeX defines inline- and display-maths commands, apparently
+analagous to those that derive from the TeX primitive maths
+sequences bracketing maths commands with single dollar signs (or pairs
+of dollar signs).
+<p>As it turns out, LaTeX's inline maths grouping, 
+<code>\</code><code>(</code><code> ... </code><code>\</code><code>)</code>, has precisely the same effect as the
+TeX primitive version <code>$ ... $</code>.  (Except that
+the LaTeX version checks to ensure you don't put <code>\</code><code>(</code> and
+<code>\</code><code>)</code> the wrong way round.)
+<p>In this circumstance, one often finds LaTeX users, who have some
+experience of using Plain TeX, merely assuming that LaTeX's
+display maths grouping <code>\</code><code>[</code><code> ... </code><code>\</code><code>]</code> may be replaced by
+the TeX primitive display maths <code>$$ ... $$</code>.
+<p>Unfortunately, they are wrong: if LaTeX code is going to patch display
+maths, it can only do so by patching <code>\</code><code>[</code> and <code>\</code><code>]</code>.  The most
+obvious way this turns up, is that the class option <code>fleqn</code>
+simply does not work for equations coded using 
+<code>$$ ... $$</code>, whether you're using the standard classes
+alone, or using package <i>amsmath</i>.  Also, the <code>\</code><code>[</code> and
+<code>\</code><code>]</code> has code for rationalising vertical spacing in some extreme
+cases; that code is not available in <code>$$ ... $$</code>, so if you
+use the non-standard version, you may occasionally observe
+inconsistent vertical spacing .
+<p>There are more subtle effects (especially with package
+<i>amsmath</i>), and the simple rule is 
+<code>\</code><code>[</code><code> ... </code><code>\</code><code>]</code> whenever
+unadorned displayed maths is needed in LaTeX.
+<p><p>This question on the Web: <a href="http://www.tex.ac.uk/cgi-bin/texfaq2html?label=dolldoll">http://www.tex.ac.uk/cgi-bin/texfaq2html?label=dolldoll</a>
+</body>