The 7th Guest

Many puzzles use grid-like boards made of invididual squares.  I will use 
the same system for describing each such board -- columns are labelled with
letters, starting with "A" on the left, and rows are labelled with numbers,
starting with "1" at the top.  A sample grid will look something like the
following, only with more letters and numbers in most cases.
=====
- A B C D
1
2
3