When a character uses a tower shield for total cover, which squares does that protect against? Obviously the square on the other side of the shield, but what about the diagonals?
Total Cover: If you don't have line of effect to your target (that is, you cannot draw any line from your square to your target's square without crossing a solid barrier), he is considered to have total cover from you. You can't make an attack against a target that has total cover.
From the definition of total cover, you can draw lines from the diagonals that don't pass through the covered edge so it would seem to me to be partial cover if anything.
A tower shield typically grants a Medium wielder total cover against melee attacks launched by Medium foes from the square opposite the shield and from similar foes' melee attacks launched from the squares to the left and right of the square opposite the shield…
The tower shield says
Then Cover says
So to determine if the defender has cover from them, the Medium foes armed with longswords occupying the left and right squares opposite the tower shield must draw a line from the least advantageous corners of their squares to the most advantageous corner of the defender's square, and such a line will go through the tower shield, giving the defender total cover.
…But for foes making ranged attacks and using reach, it's a little different
Cover also says
Thus, if attackers are employing ranged weapons or attacking with reach, to determine if the defender has cover from their attacks, the attackers, instead, pick the line to be drawn from their most advantageous corners to the defender's most advantageous corner.
While this still leaves the defender with total cover from attacks by the typical Medium attacker on the shield's opposite side, it's unclear if adjacent foes to the left and right of that attacker that are armed with ranged weapons are likewise impaired. The typical Medium creatures to the left and right of the shield opposite side can pick a corner that causes the traced line to skirt the tower shield's border, which would normally be sufficient to grant the defender cover (therefore total cover via the tower shield), but it's only from attacks that pass through the tower shield's edge against which the defender gains total cover, and whether this is through enough is a question for the GM. Ask the GM if it's possible for a dude with a crossbow to run up to the diagonal edge of your tower shield and cap you with a crossbow by, essentially, shooting around the tower shield. If the GM rules it is possible, admire such a foe's bravery before gutting him.