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David Lemon: Basic Type 1 Hinting: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.
As far as I can see, the Type 1 Specs say nothing about negative values. Also, they say nothing about where to use 20 and where to use 21. It just says that ‘either is acceptable’. Taking a sneak peek into MyriadPro-Regular.otf, I can confirm that in this font (at least in the capital I), ghost hints are set the way Lemon describes it:First, they have a negative value (thus represented in Fontographer as going from the top down instead of from the bottom up, or from right to left instead of left to right), and second, they use a value of exactly 20 units (at the tops or rights of features) or 21 units (at the bottoms or lefts). This is the main reason to pay attention to hint direction; with the wrong direction, stems that aren’t ghost hints will be treated as if they were (if they happen to have the expected weight), and intended ghost hints may be treated like regular stems.