By design, the bottom bowl of a Times New Roman “B” is wider than the top bowl. With the previous interpolation, this design subtlety is apparent at 12 points. However, the bowls appear equal at 10 points, staggered again at 8 points, and aligned again at 6 points.
Additional instructions can make the bowls fall on the grid properly. First, links from the side-bearing points control the placement of the left stroke and the bottom round stroke. Next, interpolating between those links places the top round stroke in proportion to the left (straight) stroke and the bottom round stroke, like this:
Interpolating to align the bowls.
These hints align the bowls at the three smaller sizes:
Aligned bowls at 10, 8, and 6 points and 96 dpi.
This example showed just one of many ways to align the bowls. Another approach might be to link the two round strokes. However, you cannot interpolate the bottom round stroke because it is not actually between “between” the left straight stroke and the top round stroke:
Incorrect attempt to interpolate a child point not between the parent points.
Attempting this interpolation creates a circular dependency, which Visual TrueType won’t allow. A message will tell you what’s wrong, in this case interpolating between parent points:
Visual TrueType explains the interpolation error.