# \abovedisplayskip and related commands

`\abovedisplayskip`

is one of 4 commands to control the amount of glue (flexible
space) that TEX engines add before, and after, a displayed equation:

`\abovedisplayskip[=]`

*glue*`\belowdisplayskip[=]`

*glue*`\abovedisplayshortskip[=]`

*glue*`\belowdisplayshortskip[=]`

*glue*

where *glue* is some “flexible space” and the “[=]” indicates that an *optional* “=” can
follow after the command.

**usage example** `\abovedisplayskip=12pt plus3pt minus 9pt`

( the “=” sign) is
optional).

Each of the 4 commands above can be used to set (store) your preferred glue value that TEX can use when typesetting display mathematics.

#### Choosing which glues to use

As with most TEX algorithms, a full description needs to address *many* fine details
but, *to a first approximation*, TEX checks whether the last line in the paragraph and
the equation “overlap”: the outcome of that comparison determines which glue to
place above and below the equation. In practice, TEX also tests for the presence of
an equation number.

If the equation is preceded by a “sufficiently short” line the glue stored in
`\abovedisplayshortskip`

will be inserted above the equation and the glue value
stored in `\belowdisplayshortskip`

will be inserted after it. For longer lines (or
longer equations), TEX will instead insert the glue `\abovedisplayskip`

above the
equation and `\belowdisplayskip`

below it.

A detailed explanation of exactly how TEX chooses which glue to place above a
display equation (`\abovedisplayskip`

or `\abovedisplayshortskip`

) or below it
(`\belowdisplayskip`

or `\belowdisplayshortskip`

) can be found on pages 188–9 of
The TEXbook.

#### Examples

For demonstration purposes, we’ll use some “extreme” values for the various glues so
that their effect is clearly visible. We’ll choose `\abovedisplayshortskip=-20pt`

which will moves a display equation *upwards* by 20pt if that equation is preceded by
a sufficiently short line. We’ll also use `\belowdisplayshortskip=100pt`

to set the glue (100 pt) inserted below the displayed equation. Here, we’ll
use the Schrödinger equation as the basis for illustrating the effect of these
commands:

```
\[ \frac{\hbar^2}{2m}\nabla^2\Psi + V(\mathbf{r})\Psi
= -i\hbar \frac{\partial\Psi}{\partial t} \]
```

which produces the following mathematics:

\[ \frac{\hbar ^2}{2m}\nabla ^2\Psi + V(\mathbf{r})\Psi = -i\hbar \frac{\partial \Psi }{\partial t} \] The following example uses the “extreme” glue values mentioned above:

```
\abovedisplayshortskip=-20pt
\belowdisplayshortskip=100pt
\noindent A short last line...
\[ \frac{\hbar^2}{2m}\nabla^2\Psi + V(\mathbf{r})\Psi
= -i\hbar \frac{\partial\Psi}{\partial t} \]
... a short concluding line.
```

The following graphic shows the results:

As you can see, the Schrödinger equation has been shifted upwards by 20pt with 100pt of space added below it.

However, if we now use a much wider equation with `\abovedisplayshortskip=-20pt`

and `\belowdisplayshortskip=100pt`

or increase the length of the last line in our
paragraph those two glue settings are ignored because the displayed equation
“overlaps” the last line. Here, TEX now uses the glues `\abovedisplayskip`

above
the equation and `\belowdisplayskip`

below it. Here, we have not set values for
`\abovedisplayskip`

or `\belowdisplayskip`

but simply use their default
values:

```
\abovedisplayshortskip=-20pt
\belowdisplayshortskip=100pt
\noindent A short last line...
\[\int_{x^2 + y^2 \leq R^2} f(x,y)\,dx\,dy
= \int_{\theta=0}^{2\pi} \int_{r=0}^R
f(r\cos\theta,r\sin\theta) r\,dr\,d\theta.\]
... a short concluding line.
```

The following graphic shows the results:

Now we’ll typeset the the Schrödinger equation but make sure the last line is longer:

```
\abovedisplayshortskip=-20pt
\belowdisplayshortskip=100pt
\noindent A longer last line now precedes our Schr\"{o}dinger equation. As a result,
\TeX{} will use the glues \verb|\abovedisplayskip| above the equation and
\verb|\belowdisplayskip| below it, ignoring the ‘‘extreme’’ values we supplied:
\verb|\abovedisplayshortskip=-20pt| and \verb|\belowdisplayshortskip=100pt|.
\[ \frac{\hbar^2}{2m}\nabla^2\Psi + V(\mathbf{r})\Psi
= -i\hbar \frac{\partial\Psi}{\partial t} \]
... a short concluding line.
```

The following graphic shows the results:

#### Related commands

`\displaywidth`

[=]*dimension*`\displayindent`

[=]*dimension*`\predisplaysize`

[=]*dimension*

where *dimension* is a lenth in units understood by TEX. The “[=]” indicates that an
*optional* equals follows after the command.

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