It's the "slant angle" (the setting next to Italic Angle in FontLab Studio). It works only with Type1 and OpenType CFF, not TrueType.

It has both the italic angle and slant angle boxes left blank. Under that it has a check next to "Copy Slant angle to Italic angle". I'm still puzzled. How can it copy slant data if there is none showing?

Hmm ... seems that FontLab can't read the value from a font, but it works. Try entering a value into "slant angle" of an upright font, generate and test ...

“Copy Slant angle to Italic angle” just means that when the generated font is artificially slanted, this should be reflected in the Italic Angle property of the font.

I will try that at some point. Are values equal to degrees of slant or is there some other sort if measurement?

Also...thanks for your response. There's always something new to learn.

"Italic angle" is the exact value that the font file contains for "ItalicAngle". However, the Font Matrix in the file is what the renderer should use to transform each character, and that value may differ (a bit) from the Italic angle. This value is what FL calls its 'Slant' -- and notes it's only useful for Type 1.

The value for ItalicAngle should be in degrees, rotating RIGHT for a positive number (although the Slant is (apparently) negative, and changing it changes the ItalicAngle sign as well ...).