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If my manuscript file has

$$\biggl({\partial^2\over \partial x^2}+
   {\partial^2 \over \partial y^2}\biggr)\bigl|\varphi(x+iy)\bigr|^2 = 0$$

or

$$\bigg({\partial^2\over \partial x^2}+
  {\partial^2 \over \partial y^2}\bigg)\big|\varphi(x+iy)\big|^2 = 0$$

the result will be the same :

enter image description here

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to post code snippets, use 4 leading spaces, not $$. –  ℝaphink May 30 '11 at 19:26
    
@Raphink Didn't follow you. Could you give more details ? –  bellochio May 30 '11 at 19:30
2  
@Raphink $$ plain TeX for displayed math. (It's better to use \[ \] but that's a separate issue. See Why use \[...\] instead of $$ –  Alan Munn May 30 '11 at 19:31
    
@Alan: Oh sorry, I thought that was an attempt at enforcing a code higlighting by using two $ instead of a single one. My bad ;-) –  ℝaphink May 30 '11 at 19:34
1  
See also: difference between \big[ and \bigl[ –  Stefan Kottwitz May 30 '11 at 20:07
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3 Answers 3

up vote 16 down vote accepted

\big (and its friends) create an Ord(inary) atom. \bigl and \bigr (and their friends) create Open and Close atoms. Spacing between atoms can vary.

For example, TeX inserts a thin space between an Op (big operator) atom and an Ord atom, while it inserts no space between an Op atom and an Open atom.

Compare (Op and then Ord):

\[ \sum\big( \]

with (Op and then Open):

\[ \sum\bigl( \]

You can see the a list of the 13 kinds of atoms in page 158 of the TeXbook. The table in page 170 shows the spacing between pairs of adjacent atoms.

EDIT: In the example give by bellochio, there's no difference in spacing due to the kind of atoms involved in both expressions; the only part which could be considered problematic is the initial part:

\biggl({\partial^2\over \partial x^2}

versus

\bigg({\partial^2\over \partial x^2}

However, using \showlists for the first expressions produces (only the relevant parts are shown):

### display math mode entered at line 1
\mathopen
.\hbox(14.5001+9.50012)x7.36115
..\mathon
..\hbox(14.5001+9.50012)x7.36115
...\hbox(0.39998+23.60025)x7.36115, shifted -14.10013 []
...\vbox(14.5+0.0)x0.0
...\hbox(0.0+0.0)x0.0, shifted -2.5
..\mathoff
\mathord
.\fraction, thickness = default
.\\mathord
.\.\fam1 @
.\^\fam0 2
./\mathord
./.\fam1 @
./\mathord
./.\fam1 x
./^\fam0 2

so the first atoms are of type Open, and Ord. For the second expression \showlists produces:

### display math mode entered at line 1
\mathord
.\hbox(14.5001+9.50012)x7.36115
..\mathon
..\hbox(14.5001+9.50012)x7.36115
...\hbox(0.39998+23.60025)x7.36115, shifted -14.10013 []
...\vbox(14.5+0.0)x0.0
...\hbox(0.0+0.0)x0.0, shifted -2.5
..\mathoff
\mathord
.\fraction, thickness = default
.\\mathord
.\.\fam1 @
.\^\fam0 2
./\mathord
./.\fam1 @
./\mathord
./.\fam1 x
./^\fam0 2

so the first atoms are of type Ord, and Ord. According to the table in page 170, TeX inserts no space between an Open and an Ord atom and also between two Ord atoms and that's why there's no difference in the spacing.

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From the table on page 170 we have : space between an Ord atom and an Inner atom = (1). Space between an Open atom and an Inner atom = 0. How come the spaces after the first parentheses were equal in my example, in the two cases ? –  bellochio May 30 '11 at 21:30
    
@bellochio: I've updated my answer given an explanation. –  Gonzalo Medina May 30 '11 at 23:19
    
I'm giving you the credit for the answer, but there is still something I didn't understand : according to your explanation, the distance between the first parenthesis an the first partial derivative should be 0. But you can see clearly that there's at least one pixel between those two atoms.How that could be explained ? –  bellochio May 31 '11 at 0:30
    
@bellochio: I did a test boxing the symbols with a frame with \fboxsep set to 0pt and the space belong to the symbols themselves. –  Gonzalo Medina May 31 '11 at 19:24
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As has already been pointed out in the other answers, using "l" immediately following \Bigg, \bigg, \Big, and \Big informs TeX that the subsequent "fence symbol" -- (, [, \{, etc -- is to be given math-class "Math Open" rather than "Math Ordinary".

The consequences of this difference in the math class status of the fence symbol ("Open" vs "Ordinary) are particularly striking if the first math atom that follows the fence symbol is an arithmetic operator such as + (plus), - (minus), or \times:

  • If the fence was assigned class "Math Open", i.e., if \Biggl, \biggl, etc was used, TeX will -- correctly -- not insert a bit of extra space between the fence symbol and the -/+/\times symbol.

  • Absent the l (mathopen) qualifier, i.e., if the fence was assigned class "Math Ordinary", TeX will incorrectly interpret the -/+/\times symbols as binary operators (class "Math Bin") and thus also insert a bit more whitespace between these symbols and the next math atom.

The following examples illustrate the resulting differences in spacing, both between the opening fence and the arithmetic operators and between the arithmetic operators and what follows:

enter image description here

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\Biggl[-2x - 4y\Biggr]& \quad\text{\emph{with} ``l'': } 
     \begin{array}{l}
     \text{\textbullet\ tight spacing after opening fence;}\         \text{\textbullet\ first ``$-$'' symbol treated as unary operator}
     \end{array}\    \Bigg[ -2x - 4y\Bigg]&  \quad\text{without ``l'': }
     \begin{array}{l}
     \text{\textbullet\ loose spacing after opening fence;}\         \text{\textbullet\ first ``$-$'' symbol treated as binary operator}
     \end{array}\    \biggl(+3u + 7v\biggr)&\    \bigg( +3u + 7v\bigg) &\    \Bigl\vert \div 2x \times 4y\Bigr\vert &\    \Big\vert  \div 2x \times 4y\Big\vert  &\    \bigl\{\times 3u \div 7v\bigr\} &\    \big\{ \times 3u \div 7v\big\}  &
\end{align*}
\end{document}
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Is this happening even if I group the -,+ with the number that follows? –  percusse Jun 30 at 9:22
1  
@percusse -- If one encases a group of math atoms -- a math "molecule"? -- in curly braces, the resulting math atom will be of type "Math Ord" unless one explicitly assigns a different math type. Grouping the + and - symbols with the subsequent material will thus succeed in avoiding getting too much whitespace between the opening fence and the following math atom. Speaking for myself, I do find it more natural to assign math-open status explicitly to an opening fence than to have to remember to encase the interior material in curly braces. –  Mico Jun 30 at 11:07
    
Ah got it thanks. –  percusse Jun 30 at 19:09
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In that case, yes, perhaps, but try

$\log\bigl($ vs. $\log\big($

they are not the same, because \big( is AFAIR not of type math open

there are also examples regarding the type of the right fence (though I've forgotten about it at the moment)

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You mean the difference is the space between the 'g' and the '(' ? Why this difference didn't show up in my example ? –  bellochio May 30 '11 at 19:40
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