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I always use align in my documents, and avoid equation. Is there anything wrong with that? My reasoning behind this: align > equation, so why not use it?

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4 Answers

equation "squeezes" the math (i.e. uses less white space between symbols) in order to avoid badboxes but align does not.

\documentclass[11pt,draft]{article}
\usepackage{amsmath}
\begin{document}
 Using ``equation'':
 \begin{equation*}
  1+2+3+4+5+6+7+8+9+10
  = 11+12+13+14+15+16+17+18+19+20+21
 \end{equation*}

 Using ``align'':
 \begin{align*}
  1+2+3+4+5+6+7+8+9+10
  = 11+12+13+14+15+16+17+18+19+20+21\\
 \end{align*}
\end{document}

enter image description here

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I guess what you're showing is that an align[*] environment, if used mindlessly (or recklessly...), more easily produces seriously overfull lines of math than does an equation[*] environment. Of course, one rather good reason for using an align environment to begin with is to have a method for avoiding the creation such overfull lines. –  Mico Jan 20 at 6:26
    
@Mico Exactly!...which is why I was so surprised when I noticed this behavior in one of my papers. I used to be like the OP and thought align > equation used thus used align everywhere. The only argument I had seen to use equation is because you want the syntax reflect this. However, I often have multi-lined equation that need alignment and even tend to edit equation so the that the need for alignment comes and goes. I found it an annoying waste of time to change the syntax between align and equation. –  Tyson Williams Jan 20 at 14:32
    
Eventually are stated using equation for one liners just because my coauthors did. Only in the last week have I realized that there is a visible difference between these two environments (primarily this answer of mine and Will's answer about differing vertical white space before the display math). –  Tyson Williams Jan 20 at 14:34
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align is unable to replicate the label behavior of equation + split, which is to have a single vertically centered label. One can suppress labels for specific lines of align using either \nonumber or \notag, but all unaltered lines get their own label.

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\begin{document}
 Using ``equation'':
 \begin{equation}
  \begin{split}
   a+b
   &= c\\
   &= c\\
   &= c\\
   &= c
  \end{split}
 \end{equation}

 Using ``align'':
 \begin{align}
  a+b
  &= c\\
  &= c \notag\\
  &= c \nonumber\\
  &= c
 \end{align}
\end{document}

enter image description here

(Answer inspired by this source.)

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While not exactly a bad idea in principle, unfortunately it is a bad idea in practise because align doesn't have the same feature as equation whereby less vertical space is added if a small equation is displayed after a paragraph that ended early on the line. For example, consider

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\begin{document}
hello
\[
a+b=c
\]

hello
\begin{align}
a+b&=c
\end{align}
\end{document}

You should easily be able to see the extra space after the second ‘hello’.

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What are you supposed to do though if you need to use an align environment (i.e., if typeseting an equation environment with an aligned inside is not sufficient)? –  mSSM May 21 '12 at 13:11
1  
If you need to use align then you just use align :) Just be aware of the difference, that's all. –  Will Robertson May 22 '12 at 2:01
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Will all due respect to Will Robertson, actually I think that it is a bad idea in principle, because they mean different things and markup (be it *TeX, HTML,...) should be logical as much as possible.

If you are typesetting an equation, then use equation and if you desire alignment you can use aligned blocks inside the equation.

You can decide easily whether you are typesetting an equation or not, by thinking how to want to reference it. If you only want one equation number to be displayed and see yourself suppressing by hand any additional equation numbers, (say by using \notag) then you are typesetting an equation and your markup should reflect that.

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2  
I see what you mean about markup being logical and do agree with you in principle, but I think in this case the typesetting of the output is far more important than whether or not we can consider a single equation to be aligned with itself (and therefore 'logical' within an align environment). The difference in behaviour outlined by Will seems to me to provide a much better answer as to the actual difference between the two environments and where you could possibly run into trouble by choosing one over the other. –  Michael Underwood Jul 27 '10 at 16:39
6  
I interpret "not exactly a bad idea" as a polite way of saying not really a good idea. –  Charles Stewart Jul 28 '10 at 9:24
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