Alignment metrics are the heights at which the letters of a font align: the baseline, the x-height, the cap height, and any other line a design needs, such as the ascender of b, d and h, the descender of p and q, the x-height of small capitals, or the top of the Hebrew or Arabic letters. Every line has a position per master and an overshoot zone, and it can be limited to part of the font. FontCreator uses them in the glyph editor, in Set Anchors and when exporting PostScript blue zones.
The lines are edited on the Masters page of Font Properties, in the Alignment Metrics group. This topic explains what they are; that topic explains the controls.
Built-in lines
Baseline, x-Height and Cap Height are always present. Their positions are the font's own values: the baseline is 0, and x-height and cap height are the same numbers as the x-Height and Cap Height boxes above the list, so there is one x-height, not two. The list adds an overshoot, a usage and a role to them.
There is no built-in ascender or descender line. The ascender and descender values in the metrics box are line-spacing values, not the height of any letter. A line for the top of b, d, h, k and l or the bottom of p, q and g is added by Detect, comes in with a .glyphs file, or is added by hand, and then means exactly that height.
Position and overshoot
The position is the flat edge: where the top of x or the bottom of o comes to rest. Round letters go slightly past it, and the overshoot is how far. Overshoot is positive above the line (x-height, cap height: the top of o overshoots upwards) and negative below it (baseline: the bottom of o overshoots downwards). A line with an overshoot of 0 is a plain height with no zone.
The zone runs from the position to the position plus the overshoot. In the glyph editor it is drawn as a translucent band; on export it becomes a blue zone.
Scope
A line you add can be limited to a part of the font. Glyphs that do not match are not affected by it. The scope can combine:
•A script (Latin, Cyrillic, Hebrew, Arabic, ...), by OpenType script tag.
•A case: uppercase, lowercase or small caps.
•A category: letter, figure, mark, punctuation or other.
•A glyph name suffix, such as .sc or .sups.
•A list of glyph names.
A scoped line of a built-in kind replaces the built-in line for the glyphs it applies to: an x-height line scoped to small caps replaces the x-height on every small-capital glyph, and a Hebrew-scoped cap-height line replaces the cap height on Hebrew letters. Lines of other kinds are shown in addition to the built-in ones. The built-in lines themselves have no scope.
Usage
Each line is a design line or a blue zone. A design line is only a drawing aid. A blue zone is also written to the font as a PostScript alignment zone when you export OpenType CFF, variable CFF2 or UFO, and PostScript-based rasterizers use these zones to keep the overshoots from popping in and out at small sizes. Blue zones must not overlap or come too close to each other, and the format allows at most seven top zones (including the baseline) and five bottom zones; further zones are left out of the export. TrueType-based exports ignore the usage.
Role
A line can be marked as the Anchor top or Anchor bottom reference for the glyphs it applies to. Set Anchors then places the top (or bottom) anchors of those glyphs on this line instead of on the x-height or baseline. This is how a script whose marks sit on one shared line, such as Hebrew, gets that line: a Hebrew-scoped line with the Anchor top role.
Detect
Detect measures the lines from the outlines. It lists the scripts for which the font has reference glyphs, and for the script you pick it looks at a fixed set of reference letters per zone, the way the FreeType auto-hinter does: the flat letters give the position, the round letters give the overshoot. For Latin the result goes into the built-in x-height and cap height and into unscoped Ascender and Descender lines; for any other script into lines scoped to that script, created when missing. A line you edited by hand is left alone. The whole detection is one undo step.
Per master
Positions and overshoots are stored per master. An intermediate master and every instance get interpolated values, so the zones follow the weight or width of the font. With several masters selected in the list, a value that differs between them shows as Multiple, and a value you enter is applied to all of them.
In the glyph editor
With metrics visible (View > Show Metrics), the glyph editor draws the lines that apply to the glyph you are editing, with their names, and the overshoot zones as bands. With Snap to Guidelines on, points and selections also snap to the lines and to the edges of the zones.
Other formats
Alignment metrics travel with the font. A Glyphs 3 file's metrics list (positions, overshoots and filters) and a Glyphs 2 file's alignment zones are read on import and written on export. UFO reads and writes the postscriptBlueValues and postscriptOtherBlues in fontinfo.plist, and the blue zones of an OpenType CFF or a variable CFF2 font are read from the font's Private DICT when you open it. They are stored in the FontCreator project; a project saved by this version cannot be opened by earlier versions.
