Celebrimbor

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The main part of osCommerce is not constructed using grids
 
It looks basically like this:
 
grid_4 (left column) ---- grid_16 (middle part) ---- grid_4 (right column)
 
In case of only 1 column, it adjust automatically like this
 
grid_4 (left column) ---- grid_20 (middle part)
 
That mean, that you need to know in advance how many columns you plan to use, so you know what grid you are in, a 16 or a 20, I mean in case you want to use the grid system within the main content part

@@Celebrimbor,
 
even I can't see the grids you intent to use for each section, sure thing is, that there is nothing you can't do with a 24 grid that you could with a 12
 
I thing that all you have to do is, to change the grids of he left column, if even necessary

@@Celebrimbor
 
Maybe you explain first what you want to achieve, so I can be more concrere, others of course too.
 
Till now, all you said is that you want 12 columns
 
Anyway, you could also take a look to here and here about how to change the column width

Why you want a 12 column?
 
The 960gs affect only the 2 columns and main body, so if you want the columns wider, just let them be more grids.