glyph information in troff

[utzoo!watmath!rday]

  Since this is a very obscure question, I won't go into a detailed
introduction; anyone who may know the answer will know what I am
talking about.  The glyphs used in ditroff (and in the old vfont
library) have values for up, down, left and right distances of the
edges of the glyph from the origin.  Negative values for left or down
meant that the edge was actually to the right or up, respectively,
from the origin.  It seems that, if all values are positive, the
width of the character is equal to left + right, while the height
is equal to up + down (fairly reasonable).
  The problem occurs when one of these values is negative; the resulting
height (or width) is equal to up + down + 1 (or left + right + 1).
Is there some rationale for this?  Has anyone else noticed this?
Any info will be greatly appreciated.
	Thanx in advance.

				Rob Day
				M.F.C.F.
				U. of Waterloo