3D diagonals

[Markov Spiked Chain]

Have there every been any rules clarifying diagonals in 3D? For a game where reach and flight become so important, this must come up a lot.

Simply:
Does 5' reach threaten the square 5' forward, right and down, sharing only a corner with the square/cube you occupy?

Relatedly:
Does creature with 10' reach 10' up threaten the same 25'x25' area on the ground as someone standing on the ground (but without a "dead zone" since even the square directly down is 10' away)?

Assuming there's been no official ruling, how do most folks play this?

[Drone]

Yes to both. Although the rules usually only mention squares (and thus only the 2D construct), it isn't a stretch to apply the same logic to 3D cubes.

So a creature with 5 feet of reach also threatens the 5-foot cubes above each square he can reach at ground level. Just imagine rotating his reach 90 degrees from the horizontal to the vertical and you've got it.

Our group has had the same questions about AOE spells that have a radius, like fireball. How high does a fireball spell reach? My ruling was that since it is a radius, it has the same radius going up as it does going horizontally. Just rotate the radius 90 degrees to vertical.

Illustrated if a character is C, his reach would be X

Horizontal Vertical

XXX XXX
XCX XCX
XXX --- Ground

[Ascalaphus]

To start with, the game doesn't actually use 5ft cubes. Unlike width and length, height isn't gridded. Perhaps because most humanoids are much taller than they're wide?

That said, it's quite impractical to work with, so go ahead and use cubes if you need them. They're the easiest solution. They allow you to keep using the same distance measures you were previously using.