In a displayed mathematical equation with more than one component, how much space should be placed between the components?

Here are the guidelines I use, with examples in LaTeX. Recall that a `\quad`

is approximately the width of a capital M and `\qquad`

is twice the width of a `\quad`

.

## Case 1. Equation with Qualifying Expression

An equation or other mathematical construct is separated from a qualifying expression by a `\quad`

. Examples:

When the qualifying expression is a prepositional phrase it is given standard sentence spacing. Examples:

The first example was typed as (using the `equation*`

environment provided by the `amsmath`

package)

\begin{equation*} \min_x c^Tx \quad \text{subject to $Ax=b$, $x\ge 0$}. \end{equation*}

Here, the qualifying phrase is placed inside a `\text`

command, which jumps out of math mode and formats its argument as regular text, with the usual interword spacing in effect, and we re-enter math mode for the conditions. This is better than writing

\min_x c^Tx \quad \text{subject to} ~Ax=b, ~x\ge 0.

with hard spaces. Note that `\text`

is a command from the `amsmath`

package, and it is similar to the LaTeX command `\mbox`

and the TeX command `\hbox`

, both of which work equally well here.

## Case 2. Equation with Conjunction

When an equation contains a conjunction such as *and* or *or*, the conjunction has a `\quad`

on each side. Examples:

In the second example, one might argue for a `\quad`

before the qualifying “for”, on the basis of case 1, but it I prefer the word spacing. This example was typed as

\begin{equation*} a = \sum_{j=1}^n c_j v_j \quad \text{where} \quad \text{$c_j = \langle a, u_j\rangle$ for $j=1,2,\dots,n$}. \end{equation*}

## Case 3. Multiple Equations

Two or more equations are separated by a `\qquad`

. Examples:

## Limitations

It is important to emphasize that one might diverge from following these (or any other) guidelines, for a variety of reasons. With a complicated display, or if a narrow text width is in use (as with a two-column format), horizontal space may be at a premium so one may need to reduce the spacing. And the guidelines do not cover every possible situation.

## Notes

My guidelines are the same ones that were used in typesetting the Princeton Companion to Applied Mathematics, and I am grateful to Sam Clark (T&T Productions), copy editor and typesetter of the Companion, for discussions about them. Cases 1 and 3 are recommended in my Handbook of Writing for the Mathematical Sciences (2017).

The SIAM Style Guide (link to PDF) prefers a `\qquad`

in Case 1 and `\quad`

in Case 3 with three or more equations. The AMS Style Guide (link to PDF) has the same guidelines as SIAM. Both SIAM and the AMS allow an author to use just a `\quad`

between an equation an a qualifying expression.

In the TeXbook (1986, p. 166), Knuth advocates using a `\qquad`

between an equation and a qualifying expression.