The Adobe Type 1 Font Format book (PDF: 445 KB) - Adobe Partners

Figure 6a. Character level stem hints (horizontal, vertical, and ghost
stem hints)
hstem
hstem
hstem
vstem
vstem
vstem
hstem
hstem
Horizontal and vertical stems Ghost stems
hstem
hstem
Some hstem hints are necessary for interaction with the alignment
zones declared through the BlueValues and OtherBlues
hints. In order for a character’s vertical features to be considered
for alignment control, that character must have an hstem hint
specified at or in an alignment zone. In some characters this happens
naturally. For example, in a capital letter “E,” there are at
least three hstem hints declared for the three horizontal stems.
The top stem’s hstem hint should reach up to the cap-height, and
the bottom stem’s hstem hint should extend down to the baseline.
These two hstem hints interact naturally with normal
alignment zones.
In a sans serif capital letter “I”, however, there are no horizontal
stems for these hstem hints. In order to have the cap-height and
baseline alignments apply to this character as well, the character
needs hstem hints for non-existent horizontal stems at these
positions also. These so-called “ghost” stems must be created
with a stem height of 20 or 21—either is acceptable. They must
describe a y coordinate range that is inside the y coordinate range
of the character, not above a top nor below a bottom.
An hstem hint may not have its top at or in a top-zone and have
its bottom at or in a bottom-zone. In the capital “I” example,
there cannot be just one hstem hint that stretches from baseline
to the cap-height line; instead one hstem should be at the capheight
and another should be at the baseline.
Chapter 6: CharStrings Dictionary 57