[OS X TeX] Using align with "or"

[Michael Sharpe]

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