# How to write a perfect equation parameters description?

A symbols or parameters description is shown below, however, the = and the details of parameter are not aligned.

I want to make a parameters list for a equation like this, in which symbols, = and detail information are aligned in terms of vertical position.

where
...
P   =   notional permeability factor
N   =   number of waves
Sd  =   damage level
...


A tabular environment will produce more spacing before and after text. So how to make the = vertical aligned?

The codes is provided:

$$\frac{H_s}{\Delta D_{n50} } = 1.0~ P^{0.13}~ \left(\frac{S_d}{N} \right)^{0.2} \xi_m^P~ \sqrt{\cot \alpha}$$
where:

$H_s$ = significant wave height, equal to the average of the highest 1/3 of the waves

$\Delta$  = relative buoyant density, equal to $\rho_r / \rho_w - 1$, where     $\rho_w$ is the water density

$D_{n50}$ = nominal  diameter defined in Equation (2)

$P$ = notional permeability factor

$S_d$ = damage level

$N$ = number of waves

$\xi_m$ = breaker parameter based on mean wave period $T_m$

$\alpha$ = slope angle

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There is a package for that: nomencl. Actually two I know of: nomentbl. Or you could even try glossaries. It really depends on how much effort you are willing to invest (i.e. writing a book vs short paper). – Count Zero Jan 29 '13 at 10:34
I'd just use align* and \text for the stuff after the = – daleif Jan 29 '13 at 10:34
@daleif, but how to align = in a fixed vertical position – KOF Jan 29 '13 at 10:37
Not understood, don't you know align? &= – daleif Jan 29 '13 at 11:13

Define your own environment for this; here I realize it as a two column alignment; the first column is typeset in math mode, the second one in text mode; the = is added automatically, with the correct spacing.

\documentclass{article}
\usepackage{array}

\newenvironment{conditions}
{\par\vspace{\abovedisplayskip}\noindent\begin{tabular}{>{$}l<{$} @{${}={}$} l}}
{\end{tabular}\par\vspace{\belowdisplayskip}}

\begin{document}
An equation just to start
$$P+N=S_{d}$$
where:
\begin{conditions}
P     &  notional permeability factor \\
N     &  number of waves \\
S_{d} &  damage level
\end{conditions}

\end{document}


If your conditions are overlong, then you can use a different environment, that I call conditions*, based on tabularx:

\documentclass{article}
\usepackage{array,tabularx}

\newenvironment{conditions*}
{\par\vspace{\abovedisplayskip}\noindent
\tabularx{\columnwidth}{>{$}l<{$} @{${}={}$} >{\raggedright\arraybackslash}X}}
{\endtabularx\par\vspace{\belowdisplayskip}}

\begin{document}
An equation just to start
$$P+N=S_{d}$$
where:
\begin{conditions*}
P    &  notional permeability factor and something
longer that needs to be taken at the next line\\
N     &  number of waves \\
S_{d} &  damage level
\end{conditions*}

\end{document}


## Variant for variable symbols

If different symbols are needed instead of = in each line, here's how.

\documentclass{article}
\usepackage{array,tabularx}

\newenvironment{conditions}
{\par\vspace{\abovedisplayskip}\noindent
\begin{tabular}{>{$}l<{$} @{} >{${}}c<{{}$} @{} l}}
{\end{tabular}\par\vspace{\belowdisplayskip}}

\newenvironment{conditions*}
{\par\vspace{\abovedisplayskip}\noindent
\tabularx{\columnwidth}{>{$}l<{$} @{}>{${}}c<{{}$}@{} >{\raggedright\arraybackslash}X}}
{\endtabularx\par\vspace{\belowdisplayskip}}

\begin{document}
An equation just to start
$$P+N=S_{d}$$
where:
\begin{conditions}
P     & =       &  notional permeability factor \\
N     & \sim    &  number of waves \\
S_{d} & \propto & damage level
\end{conditions}

An equation just to start
$$P+N=S_{d}$$
where:
\begin{conditions*}
P     & =       & notional permeability factor and something
longer that needs to be taken at the next line\\
N     & \sim    & number of waves \\
S_{d} & \propto & damage level
\end{conditions*}

\end{document}


In order to have no break after “where:”, here's a variant of the first solution:

\documentclass{article}
\usepackage{array}

\newenvironment{conditions}[1][where:]
{#1 \begin{tabular}[t]{>{$}l<{$} @{${}={}$} l}}
{\end{tabular}\\[\belowdisplayskip]}

\begin{document}
An equation just to start
$$P+N=S_{d}$$
\begin{conditions}
P     &  notional permeability factor \\
N     &  number of waves \\
S_{d} &  damage level
\end{conditions}
Some text after the equation.

\end{document}


The conditions environment has an optional argument for changing the fixed where:; so, for instance, \begin{conditions}[with:] will use “with:”.

-
The \noindent is a little bit right aligned, when using \indent it has more spacing compared to the original \indent value. – KOF Jan 29 '13 at 13:54
I did it on purpose; if you want to have the same indentation, add @{\hspace{\parindent}} at the start of the argument to tabular (or tabularx): \begin{tabular}{@{\hspace{\parindent}>{$}l...} – egreg Jan 29 '13 at 14:17 Besides, how to make the line spacing same as the text line spacing? – KOF Jan 29 '13 at 15:14 @KOF It is the same line spacing as in the text. – egreg Jan 29 '13 at 15:17 @Fiztban I added it. – egreg Jan 29 '15 at 13:05 In this case, where there are lots of text on the right, tabular with symbol separation would have been sufficient (and simpler): \begin{tabular}{l @{$=$} l}$P$& notional permeability factor\$N$& number of waves\$S_d$& damage level \$f(x,y,z)$& distribution \end{tabular}  Resulting in It should be possible to replace @{$=$} with !{=} since the ! adds space before and after the separator but I can't get it work. Ps: thanks to siteduzero.com - Isn't this basically the same as @egreg's solution? – clemens Aug 6 '13 at 13:11 Another option is to use the tabbing environmnent: $$\frac{H_s}{\Delta D_{n50} } = 1.0~ P^{0.13}~ \left( \frac{S_d}{N} \right)^{0.2} \xi_m^P~ \sqrt{\cot \alpha}$$ where: \begin{tabbing} \phantom{$D_{n50}\ $}\= \kill$H_s$\> = significant wave height, equal to the average of the highest 1/3 of the waves\\$\Delta$\> = relative buoyant density, equal to$\rho_r / \rho_w - 1$, where$\rho_w$is the water density\\$D_{n50}$\> = nominal diameter defined in Equation \eqref{eq:dn50g50}\\$P$\> = notional permeability factor\\$S_d$\> = damage level\\$N$\> = number of waves\\$\xi_m$\> = breaker parameter based on mean wave period$T_m$\\$\alpha$\> = slope angle\\ \end{tabbing}  The first line in the tabbing environment will set the spacing (i.e. tabstop). The \kill command suppresses the line it precedes, otherwise you would have an extra space between 'where:' and the first definition. Alternatively, instead of \phantom{$D_{n50}\ \$} (the longest item on your list), you can use a fixed width: \hspace{3cm}.

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thanks, that's great. – KOF Jan 29 '13 at 11:33

As daleif suggested you could use the align* environment. You get the alignment just like in tables with '&'. Here is an example:

\documentclass{article}
\usepackage[fleqn]{amsmath}
\begin{document}

\begin{align*}
H_s &= \text{significant wave height, equal to the average of the highest 1/3 of
the waves}\\
\Delta &= \text{relative buoyant density, equal to }\rho_r\text{ / }\rho_w -
1\text{, where }\rho_w\text{ is the water density}\\
D_{n50} &= \text{nominal  diameter defined in Equation \eqref{eq:dn50g50}}\\
P &= \text{notional permeability factor}\\
S_d &= \text{damage level}\\
N &= \text{number of waves}\\
\xi_m &= \text{breaker parameter based on mean wave period }T_m\\
\alpha &= \text{slope angle}
\end{align*}
\end{document}


This should work for you. This is the output:

If you don't want all equations to be aligned left, you can use the flalign* environment, like suggested here.

-
thanks Dave, the symbols should be \flushleft though – KOF Jan 29 '13 at 11:30

I think you can try fairly easy to use alignat environment:

\documentclass{minimal}
\usepackage{amsmath}
\begin{document}

\begin{alignat*}{3}
&A   &= &e^{\pi}  \\
&AB  &= &\sqrt{x} \\
&ABC &= & \text{An equation with three letters on the left!}
\end{alignat*}

\end{document}


Resulting in

-
Note that the spaces around the = sign are much too small. – Hendrik Vogt Jan 29 '13 at 11:25
@HendrikVogt, You are right! I noticed and searched for the reason. Based on this answer, alignat doesn't add space between the columns. – Pouya Jan 29 '13 at 11:33
Just write &{}={}& instead af &=& each time to fix the spacing. – Andrew Swann Feb 6 '13 at 11:03