Originally Posted by potter

The grid is for symmetrization. There is no "10th grid, 10th column". The point of the grid is so that every element is a specific size.

For example, if each column is set to 20px in the PSD. That means if you make a three column layout. One column could be 160 pixels, and the other two 400 pixels. They are all multiples of the 20 pixel. If there are 20 "columns" in the PSD. You're not actually creating a 20 column layout. Those columns are only there to "snap" your ACTUAL columns and elements to. They're there to make everything proportional.