What's the term for the inner perimeter of something?

Say I have a building with a sidewalk on its outside. To walk it I might say something like:

"I'm going to walk the perimeter of the building."

Now say I want to walk around the perimeter of the building again, but this time I'll be inside the building. How would I express the notion of "inner perimeter" succinctly? Is there a single word to describe it?

Solution 1:

Since you are inside a room, the normal word to describe this would be simply "perimeter":

I'm going to walk around the perimeter of the room.

Or, for another common form:

I'm going to walk the perimeter of the room.

However, using the term perimeter with building always means the outside perimeter, so to refer to the inner perimeter, one must either say "inner perimeter", or use the word "room".

Solution 2:

This question presupposes a false distinction.

If the perimeter was a line drawn on a flat surface (cricket pitch, for example), a pedant might say you could only walk round it if every footstep fell directly on the line.

If the perimeter was, say, a line of close-set spikes sticking three inches out of the ground so you couldn't actually walk exactly on them, "walking the perimeter" would apply equally to just inside or just outside the line (or indeed randomly crossing over the line sometimes, thus including both).

It just so happens you can't normally walk the inner perimeter of a building, because there are usually dividing walls in the way. So by default it's assumed you mean the outer perimeter. If it is physically possible, and you want to say you're going to walk the inner perimeter, just say that. There is no more succinct way to express the difference.

Solution 3:

I don't think you can do better than inner perimeter. If you have a sense of humor, you can use E-ring -- the Pentagon's inner perimeter ring.