# How does Phasing interact with the Z-axis?

The glossary entry for Phasing is rather sparse, and doesn't seem to address any specific issues that could arise while phasing.

Phasing: While a creature is phasing, it ignores difficult terrain, and it can enter squares containing blocking terrain, obstacles, and enemy creatures.

Presumably, floors and ceilings are blocking terrain, just like walls, so it follows that a creature with phasing would ignore those squares. However, if a creature without flying starts phasing, it would seem that the creature is no longer standing on solid ground and would begin to fall as normal, ie: towards the center of the world.

Therefore, it seems that phasing is more of a "selective" phasing, that it can ignore, rather than must ignore. Is this the case, or must phasing creatures ignore all terrain?

How does this interact with actually wanting to move through 3D space, though? Can a creature move diagonally up through a wall, selectively treating parts of the intervening wall space as stairs or a ladder?

Can a creature selectively phase part of it's body? Can it stick its head through the floor to see what's below, but maintain its purchase on the floor with its hands and knees?

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D&D is hardly alone in this -- this particular wrinkle is often ignored in, like, every SF show ever. :) See further the TV Tropes article on intangibility – starwed May 19 '13 at 18:18
@okeefe The problem here is not explaining how phasing works, but adjudicating what actions are possible within the game framework. – starwed May 19 '13 at 18:58
I rather like how it was handled in Jim Butcher's Ghost Story. New ghost and experienced ghost climb into the back seat of a car... the human driver starts driving off. They're going down the highway at a good 70 mph, when suddenly new ghost says, "Wait, what's keeping us in the ca-" and suddenly falls out the back of the car. Experienced ghost reaches back, catches him by the scruff of the neck, hauls him back inside, holds him over the seat. "Stop thinking about it." – Wolfman Joe May 21 '13 at 15:44
I appreciate the cinematic problems with Phasing, but my problem is much more game specific: the player says "I'm phasing, so I do this" and the rule has to be adjudicated. I suppose "the DM will just have to make something up, the rules don't provide any guidance" is a valid answer (and may be the only one) but I was hoping for some more concrete explanation of what the rules allow a phasing player to do... – Soulrift May 21 '13 at 18:42