(the header)

Suppose those red columns are on the body.

I can make a child header and a child footer span across the whole page.

But if I want things inside the header and footer—say, a navigation bar, or a bunch of link lists—to line up with the overall page grid, then I have to try to replicate the grid inside the header and footer. Or inside an element that’s nested inside another element inside the header or footer.

It would be a lot easier to just refer to the page grid directly as a subgrid.