# Spacing of consecutive equation versus a single gather

I know the spacing produced by the following two approaches is slightly different. So my question is which one should I choose? Is it about personal preference?

## Code

\documentclass[preview,multi,border=12pt]{standalone}
\usepackage{amsmath}

\begin{document}

\begin{preview}
The following equations are too simple,
$$v_t=v_0+at$$
$$s=v_0t+\tfrac{1}{2}at^2$$
$$v_t^2=v_0^2+2as$$
where $a$ is the acceleration.
\end{preview}

\begin{preview}
The following equations are too simple,
\begin{gather}
v_t=v_0+at\\
s=v_0t+\tfrac{1}{2}at^2\\$$v_t^2=v_0^2+2as$$\end{gather}
where $a$ is the acceleration.
\end{preview}

\end{document}

-
Simply looking at the result shows that the separate equation environments are wrong. Can you see the different interline spacing? – egreg Nov 5 '12 at 12:40
@egreg : interline spacing between the equations? – kiss my armpit Nov 5 '12 at 12:42
personally I would prefer alignat which gives same spacing as gather. – mythealias Nov 5 '12 at 12:43
Look between equation (1) and (2), and between (2) and (3). No way this can be "personal preference": it's wrong spacing. – egreg Nov 5 '12 at 12:43
@egreg: It should be regarded as unbalanced spacing (rather than wrong spacing) I think. – kiss my armpit Nov 5 '12 at 12:45

You definitely shouldn't do it with 3 consecutive equations. As you said yourself, the spacing is unbalanced. Here's why: The line before the first equation is "long", so TeX uses \abovedisplayskip above it, and \belowdisplayskip below. The other two equation don't have text before them, or you could say the text before the equations is "short". Thus, TeX uses \abovedisplayshortskip and \belowdisplayshortskip. As a result, you have

• \belowdisplayskip + \abovedisplayshortskip between the 1st and the 2nd equation, and
• \belowdisplayshortskip + \abovedisplayshortskip between the 2nd and the 3rd equation.

I'd use align (and not gather) to obtain horizontal alignment and balanced vertical spacing:

\begin{align}
v_t &= v_0 + at \\
s &= v_0t + \tfrac{1}{2} at^2 \\
v_t^2 &= v_0^2 + 2as
\end{align}

-
What convention do we usually adopt to decide whether or not we need to use gather instead of align for consecutive equations? I am confused to make such a decision. I know it is about horizontal alignment for sure. – kiss my armpit Nov 5 '12 at 13:06
If you have "natural" points for horizontal alignment, then use align. An 0 or \le sign is such a "natural" point. – Hendrik Vogt Nov 5 '12 at 13:30

The line

s=v_0t+\tfrac{1}{2}at^2


produces a fraction which here does not give you a higher line. If you used a \frac{}{} instead, it might happen that one line will be higher than the others (if they are wo fractions), which is also unnatural. Then it helps to put \phantom{\frac{1}{2}} on the end of the other lines (after the dot or comma sign). At least then, even with consecutive equations, which I don't recommend, the spacing would be balanced.

-