Pete said:
So, if I understand correctly...

The grid is simply lines drawn at 15 minute intervals latitude and
longitude, with each box in the grid labeled starting from the first
complete box found on the sectional in the upper left corner?

I get 34 boxes across for the Seattle sectional, by 20 boxes down.

Pete,

That's pretty close...the problem comes when you get to the overlap in
sectionals. The same piece of real estate would potentially have two grid
numbers associated with it, depending upon which sectional you were using.
For example....here in Alabama we may be on the New Orleans or the Atlanta
sectional. There's a rule for assigning grid numbers for those
"over-lapped" grids. This is one of the reasons that the gridding system is
confusing, at least not crystal clear, for you must keep your grid
conversion sheet handy.

With the "new" system, the grid is named by the lat/long on the lower
right-hand corner of the degree intersection. The quadrants of the total
grid are still referred to as "A-B-C-D" and those can be further broken down
into "Aa, Ab, Ac, Ad....." That gets you down to the 7.5 minute squares.

Stephen