# Left-align all columns in equation

Just expanding a bit on @vonbrand's answer ...

The `alignat`

environment is an extension of the `align`

environment. It uses the `&`

alignment symbol to align the blocks in alternating left and right manner.

If two left-aligned blocks are supposed to follow each other, use

`&&`

rather than`&`

.You can create extra space between two left-aligned blocks by inserting

`\quad`

or`\qquad`

between the consecutive`&`

symbols.The

`alignat`

environments requires an argument, an integer. To calculate this integer, take the maximal number of`&`

symbols in any row, add 1, and divide by 2. In the example below, the maximal number`&`

symbols is 5; adding 1 and dividing by 2 gives 3. (If the maximal number is an even number, add 1 before proceeding.)

Some additional comments: I would replace all instances of `...`

with `\dots`

in order to create typographical ellipses; and I'd encase the 3 `:`

symbols in curly braces in order to avoid the extra spacing that otherwise gets inserted.

```
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat}{3} % max. # of '&' symbols in any row: 5. (5+1)/2 = 3
& a[i{:}j] &&= \langle\,\rangle
&\qquad&\text{if $i > j$} \\ % or '\quad', if you prefer
& a[i{:}j] &&= \langle a_i, \dots, a_j \rangle
&&\text{if $a = \langle a_0, \dots, a_i, \dots a_j, \dots \rangle$} \\
& a[i{:}] &&= \langle a_i, \dots \rangle
&&\text{if $a = \langle a_o, \dots, a_i, \dots \rangle$}
\end{alignat}
\end{document}
```

Use `alignat`

, and read the documentation carefully. It has quite funny ideas of how to align column contents, you have to pick the ones to fill with some care. A bit of experimentation (and some dummy columns, perhaps for spacing only) will get you the results you desire.