In Great Britain, we split the country into 100km squares for the National Grid system.

When you opt to put grid numbers on the map, they give you the Eastings and Northings within a grid square, but which square?

We populate the grid square in the header:

The grid numbers on the above map printout are therefore in the square NH.

However, its possible to be on the edge of the 100km square, and in which case you will get a more complex title such as:

This is simply telling you that the 

(TL) Top Left is in square NG. 

(TR) Top Right is in square NH.

(BL) Bottom Left is in square NM.

(BR) Bottom Right is in square NN.

If you are across just two squares, you will get Top and Bottom, or Left and Right depending on where the split it.