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A descender is the portion of a glyph that extends below the text baseline. Lowercase “p”, “q” and “y” commonly have one; the exact form depends on the typeface. A descender describes the visible shape relative to the writing reference, rather than the distance through which the logical pen position advances.^[ref]

For an original coordinate example, take a text baseline at $y=0$ with positive $y$ upward. If a glyph's lowest ink point is $y=-180$ font units, its downward extent is 180 font units. At a scale of 20 pixels per em with 1000 font units per em, this becomes 3.6 pixels before grid fitting. The extent is positive even though the lowest coordinate is negative.

The font descent is a related global layout metric, with sign conventions that depend on the format or API. It need not equal the downward extent of each individual descender. Line height and neighbouring ink extents together determine whether forms on successive lines collide; the nominal font size alone does not decide that.

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 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.

The x-height is the height of the main lowercase body of a typeface, conventionally associated with the letter “x”. It excludes the extra extent of ascenders and descenders; the cap height describes a different reference. Small optical overshoots mean that not every lowercase outline without an ascender has exactly the same ink height.^[ref]

Write $r$ for the x-height as a fraction of an em. At nominal scale $p$ pixels per em, its ideal scaled height is $rp$ pixels, before grid fitting. Equal font sizes do not force equal $r$, so changing the typeface can alter the apparent lowercase size.

To preserve a target height $h$ while switching to a design with ratio $r_2>0$, choose $p_2=h/r_2$. If the original ratio was $r_1$ at scale $p_1$, this becomesFor a hypothetical switch from $r_1=0.5$ at 16 pixels per em to $r_2=0.4$, the matching scale is 20 pixels per em. Both lowercase bodies then have ideal height 8 pixels. This is the geometry behind metric-based font-size adjustment when a fallback font replaces another.^[ref]