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The font descent supplies the below-text baseline part of a font's vertical layout metrics. It must be distinguished from the descender of an individual glyph. The coordinate convention also matters: a downward extent can be represented as a negative coordinate in a system whose vertical axis points upward, or as a positive distance in another API.^[ref]

For example, a signed descent $d_f=-200$ font units represents a downward extent $D=200$ font units. With 1000 font units per em and 20 pixels per em, its magnitude scales to 4 pixels. Calling that a negative height would confuse the coordinate with the distance being allocated.

Using a signed descent, an ascent $a=800$ and a font line gap $g=200$ in the same font units givesEquivalently, if $D=-d_f$ is the positive downward extent, the same line height is $h=a+D+g$. This is the elementary typographic spacing model; the rendering system's choice of metrics must still be respected.^[ref]

A computer font can contain several descent-related fields. For example, OpenType's hhea header stores a typographic descender; an application must not infer the sign and purpose of every other API field merely from the word descent.^[ref]

A font metric is a numerical measurement used to position text from a computer font. Measurements include glyph advances, glyph bearings, bounding boxes and face-level quantities such as font ascent, font descent and font line gap. These describe both logical movement and the geometry of the visible shapes.^[ref]

For a horizontal example, put the pen position at $x=0$, let a glyph have advance 12, and let its ink extend from $x=-2$ to $x=9$. Its ink width is $9-(-2)=11$, while the next pen position is $x=12$. The negative left glyph bearing describes an overhang; it does not imply a negative glyph advance.

A separate example illustrates vertical conventions. With font ascent $a=12$, signed descent $d=-3$ and font line gap $g=2$, the metric-based baseline separation isSome font formats store the descent magnitude with the opposite sign, so the convention must be checked before using the formula.^[ref] These measurements inform layout before rasterization determines the actual pixels.

A glyph bearing is a signed offset locating a glyph's ink relative to the reference used for layout. In a simple horizontal metric, the left side bearing measures the offset from the pen position to the left ink edge, and the right side bearing measures the remaining distance from the right ink edge to the advance endpoint.^[ref]

Put the origin at $x=0$, the glyph advance at $a$, and the ink edges at $x_L,x_R$. ThenFor $x_L=-2$, $x_R=9$ and $a=12$, the bearings are $-2$ and 3, while the ink width is 11. The negative left bearing records an overhang across the origin. The next nominal pen position remains at $x=12$.

These are measurements of a default horizontal arrangement. Subsequent glyph positioning and kerning can change the rendered relationship between neighbouring glyphs. The reference point and axes must also be checked when applying bearing conventions to vertical layout.

In a simple unpositioned horizontal layout, the left side bearing is the signed distance from a glyph's pen position to the left edge of its ink bounding box. Put the logical origin at $x=0$ and call the left ink edge $x_L$. Then $\mathrm{lsb}=x_L$. A positive value leaves space before the ink; a negative value means that the ink overhangs the origin.^[ref]

For example, a glyph whose ink occupies $40\le x\le 540$ and whose glyph advance is 600 font units has left side bearing 40, ink width 500 and right side bearing 60. These are default metrics; subsequent glyph positioning can change the rendered arrangement.

OpenType's horizontal-metrics table supplies the left side bearing and glyph advance. One must not assume that every outline's stored xMin is already the left side bearing: the outline's own coordinate origin and the metrics' logical origin need not coincide in every supported format.^[ref]

In glyph positioning, the pen position is the logical point from which the next glyph is placed. The word pen describes a layout convention: the renderer can draw an outline or a collection of pixels rather than trace a physical pen stroke. Simple horizontal layout places this reference point on the text baseline.^[ref]

Let ${\mathbf{p}}_i$ be the current pen position, ${\mathbf{o}}_i$ a placement offset and ${\mathbf{a}}_i$ the resolved glyph advance. The current glyph's drawing origin and the next logical point are, respectively,For example, in one horizontal coordinate system, ${\mathbf{p}}_i=(10,20)$, ${\mathbf{o}}_i=(-1,2)$ and ${\mathbf{a}}_i=(8,0)$ draw at $(9,22)$ but leave the next logical point at $(18,20)$. Moving the current shape and changing the progression of the run are separate operations.

A glyph's bounding box is therefore not a substitute for its pen position. A left side bearing can leave space between the logical origin and the ink, and a right side bearing records how the ink ends relative to the default advance. Kerning or attachment of a combining character can modify the placement or progression without changing the stored outline.

A text baseline is the reference line used to align glyphs in a run of text. It supplies a placement reference rather than the edge of every visible shape. In a horizontal coordinate system with the baseline at $y=0$, a descender can extend below the line, while an ascender extends above the main body of a lowercase letter. Aligning ink bounding boxes at their bottom edges would therefore fail to align the writing.^[ref]

The reference depends on the writing system and layout. OpenType distinguishes an alphabetic baseline (romn), a hanging baseline (hang) and a mathematical baseline (math), among others. The last is the reference about which mathematical forms are centred; it need not coincide with the alphabetic baseline. Vertical layout also requires an appropriate baseline convention.^[ref]

For a simple horizontal run, start the logical pen position at $(x_0,y_0)$ on the chosen text baseline. A glyph advance $(a,0)$ moves the next pen position to $(x_0+a,y_0)$. A vertical placement adjustment can move an accent glyph while leaving that baseline and the subsequent logical positions intact. This distinction lets glyph positioning align the run while placing individual forms precisely.