# [OS X TeX] Using align with "or"

Michael Sharpe msharpe at ucsd.edu
Sun Apr 17 21:55:14 EDT 2011

On Apr 17, 2011, at 6:39 PM, Alain Schremmer wrote:

>
> On Apr 17, 2011, at 6:47 PM, Ross Moore wrote:
>
>> Hi David,
>>
>> On 18/04/2011, at 8:35 AM, David Arnold wrote:
>>
>>> Bruno et al,
>>>
>>> This is precisely what I needed. But now I think I also need an explanation. We have:
>>>
>>> \begin{alignat}{5}
>>>
>>> But the first row has 9 ampersands. I've stumbled into solutions like this on my own, but I just don't understand the use of the ampersands.
>>
>> Lee Witt said:
>>
>>>>> If you want to align n columns use 2n-1 &s
>>
>> The point is that the alignment at each ampersand alternates:
>>   right alignment, then left alignment.
>>
>> So if you have columns each with an alignment point "inside" the content,
>> then you need "&&" between the columns,
>>
>>  viz.
>>
>>    A&=B   &&   C&=D   && ...
>>
>>> Is this use of the ampersands explained in any documentation?
>>
>> Try  The LaTeX Companion    1st or 2nd editions.
>
> But Companion two does not even mention \alignat. I just looked because I had been "doing it by hand", that is with a lot of \hspaces, and wanted to know if I had missed it.
>

A good source for information on all the LaTeX math environments is mathmode.pdf. You can bring it up with

texdoc mathmode

Michael