Setting up a grid

Here is the introduction to the explicit grid section of the W3C specification (https://www.w3.org/TR/css-grid-1/#explicit-grids). It's worth rereading a few times as it is dense with essential information for understanding how the grid works:

The three properties grid-template-rows, grid-template-columns, and grid-template-areas together define the explicit grid of a grid container. The final grid may end up larger due to grid items placed outside the explicit grid; in this case implicit tracks will be created, these implicit tracks will be sized by the grid-auto-rows and grid-auto-columns properties.

The size of the explicit grid is determined by the larger of the number of rows/columns defined by grid-template-areas and the number of rows/columns sized by grid-template-rows/grid-template-columns.

Any rows/columns defined by grid-template-areas but not sized by grid-template-rows/grid-template-columns take their size from...