Why is there one alignment on left, one on right, and one on left again?

Question

In this code:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath} 
\begin{document}

\begin{align*}
&A B = C   \qquad &D E = F    \qquad &G H = I \qquad &K L M = N \\
&A B C = D \qquad &E F G = H  \qquad &I J K = L M
\end{align*}

\end{document}

why is the first column aligned on left, the second on right, and the third on left?

How to have them always aligned on left (possibly by using align*, or a built-in environment but no new package)?

Answer 1

Your “first” column is actually the second.

That's how align works: it makes pairs of right/left aligned columns, and computes the space between the pairs according to some internal rules (there's a maximum separation, which is reduced if the display is crowded).

If you only want left aligned columns, just skip the odd numbered ones:

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

\begin{align*}
&A B = C    &&D E = F    &&G H = I      &&K L M = N \\
&A B C = D  &&E F G = H  &&I J K = L M
\end{align*}

\end{document}

There is one thing to care about. If one of the expressions begins with an operator such as \sum, \prod, \sin, \log and so on, the alignment would be off on the left side; this is cured by adding \! after && (or & in the first column pair). This is because align starts each even numbered column with {} which triggers a thin space before operators.