[OS X TeX] Using align with "or"

[Alain Schremmer]

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.

Best regards
--schremmer