So this came up last night for me. Using the standard grid system I had a bad guy just inside a doorway, outside the doorway and one square off was the player. I went to hit him and he complained that the wall should give him some cover, because the complete square of the bad guy cannot see his complete square. Is there somewhere that spells this situation out plainly?
Simple text drawing...
B= bad guy
G= player
W are walls
D is the doorway
Please imagine the Wall and Doorway as just a line and does not take up a 5 foot square while the player and bad guy do take up a 5 foot square.
........G....
WWWDWWW
..........B...