Font Metrics and Glyph Metrics

A glyph's outline defines its shape, but the outline alone does not determine how text is assembled. Horizontal glyph metrics position neighboring glyphs. Vertical font metrics provide the reference heights and default space between lines. Both use font units and the same baseline-based coordinate system as the drawing.

Glyph Assembly into Text

In a simple horizontal line, each glyph starts at an origin and advances by its advance width before the next glyph is placed. The drawing occupies part of that interval. Its left sidebearing and right sidebearing describe the space between the drawing's extents and the interval's boundaries in ordinary upright measurement.

For an upright glyph whose drawing extends from x = 50 to x = 550 and whose advance width is 600 units, the left and right sidebearings are both 50 units. The width of the drawing itself is 500 units. These are three different measurements: drawing width, advance width, and the bearings around the drawing.

Sidebearings can be negative when a drawing overhangs its advance interval. A blank space glyph can have an advance width without having any outline. Neither case changes the role of the advance width in positioning text.

Kerning adds a pair-specific adjustment to this ordinary spacing. Other layout positioning can also affect the result. Start by distinguishing the glyph's own spacing from adjustments that depend on its neighbors. The how-to workflow is taught in Spacing Glyphs.

Lines need a vertical interval as well. The distance from one baseline to the next is line spacing, also called line height in many interfaces. Font-level metrics supply a default; an application can replace it with an explicit leading or line-height value.

Vertical Font Metrics

FontLab displays several horizontal reference lines in the Glyph window. Their values are y-coordinates measured from the baseline, except for Line gap, which is an additional distance.

Value	Meaning
Baseline	The zero-height reference, y = 0
x Height	The design height of typical flat lowercase letters
Caps height	The design height of typical flat uppercase letters
Ascender	The upper reference used in the font's line-spacing metrics
Descender	The lower line-spacing reference; negative when below the baseline
Line gap	Additional distance added to the ascender-to-descender span

The Font Info panel provides a compact view of common font dimensions. These example values describe one design, not recommended defaults.

The design heights are references, not commands that move an outline to a line. A round letter can extend slightly beyond a flat-letter height, and accents can rise above the cap line. Changing Caps height does not scale the capitals; changing x Height does not redraw the lowercase.

The Ascender and Descender line-spacing fields supply the ascent and descent roles mentioned in Font Structures; their labels do not make them identical to a letter's ascender height or descender depth. They can reflect the design's tall and deep forms, but they need not coincide with the highest and lowest point of every glyph. Consider the whole character set and family when choosing them. The measured bounding box tells you where the actual content reaches; these metrics express the vertical framework you intend the font to use.

Default line spacing

For the ascender, descender, and line-gap system, the baseline interval is:

Ascender − Descender + Line gap

For example, an Ascender of 800, a Descender of −200, and a Line gap of 200 give a 1200-unit baseline interval. In a 1000-UPM font, that is 1.2 em. At 10-point text size, it corresponds to 12-point baseline spacing when the application uses these metrics without an overriding line-height setting.

A zero Line gap does not mean zero space between lines. It means that the ascender-to-descender span supplies the whole interval. Likewise, increasing Line gap changes the default interval; it does not move points within the glyphs.

Applications do not all select or interpret vertical metrics in the same way. FontLab also provides Safe top and Safe bottom as family-level vertical bounds, and export settings determine how the available metrics are written. Their export treatment is covered in The Export Process and the Family Dimensions reference. Check the exported font in the applications you intend to support. For a new font, Font Info > Other Values > Prefer typo metrics expresses the preference for typographic line-spacing values in applications that support it.

Family Dimensions and Font Dimensions

This implements the family-property/font-property distinction from Naming Fonts and Families: family dimensions coordinate values across related members, while font design dimensions describe an individual design. FontLab separates these values according to the consistency they need:

• Family Dimensions contains UPM, coordinate rounding, and the principal vertical line-spacing values, including Ascender, Descender, Line gap, Safe top, and Safe bottom.
• Font Dimensions contains design proportions such as Caps height and x Height, along with parameters such as the italic angle. These can vary between masters as the design requires.

Family Dimensions contains UPM and shared vertical metrics. Select the intended master before interpreting or changing its values.

Keep the line-spacing values consistent across related family members when they need to share the same text rhythm. Coordination across masters is developed in Managing Layer Content. A line containing a bold or italic word should not acquire a different height merely because that style has a different drawing extent.

“Family” describes the intended consistency; it does not mean that changing one open font automatically updates every separate font file in the family. FontLab can also store these metrics per master. Check the master shown in Font Info, and check related masters and family files when you revise shared values. Fields can highlight differences between masters.

To establish the framework:

1. Open File > Font Info and check the active master.
2. In Family Dimensions, set the chosen Ascender and Descender references. Set Line gap so that Ascender − Descender + Line gap gives the baseline interval you intend for the family.
3. In Font Dimensions, set Caps height and x Height to the design’s nominal flat-letter heights. In a Latin design, use letters such as H and x as references rather than the tops of rounded or accented letters.
4. Apply the changes, then inspect representative lowercase, capitals, accents, and descenders in the Glyph window.
5. Check a multiline sample as well as individual glyphs.

The Font Dimensions label in the View menu is broader than the page of the same name: it controls the display of dimension lines from both groups. Their appearance on the canvas does not tell you which Font Info page owns the value. Use the line's meaning — family line spacing or design height — to choose the right setting.