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<title>Ligatures and Kerning (The GNU Troff Manual)</title>
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<div class="subsection-level-extent" id="Ligatures-and-Kerning">
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<h4 class="subsection" id="Ligatures-and-Kerning-1">5.19.8 Ligatures and Kerning</h4>
<a class="index-entry-id" id="index-ligatures-and-kerning"></a>
<a class="index-entry-id" id="index-kerning-and-ligatures"></a>
<p>Ligatures are groups of characters that are run together, i.e, producing
a single glyph. For example, the letters ‘f’ and ‘i’ can form a
ligature ‘fi’ as in the word ‘file’. This produces a cleaner look
(albeit subtle) to the printed output. Usually, ligatures are not
available in fonts for TTY output devices.
</p>
<p>Most PostScript fonts support the fi and fl ligatures. The C/A/T
typesetter that was the target of <abbr class="acronym">AT&T</abbr> <code class="code">troff</code> also
supported ‘ff’, ‘ffi’, and ‘ffl’ ligatures. Advanced typesetters or
‘expert’ fonts may include ligatures for ‘ft’ and ‘ct’, although GNU
<code class="code">troff</code> does not support these (yet).
</p>
<p>Only the current font is checked for ligatures and kerns; neither
special fonts nor special charcters defined with the <code class="code">char</code> request
(and its siblings) are taken into account.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_002elg"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.lg</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">flag</i></span>]</var><a class="copiable-link" href='#index-_002elg'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-lg"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cn_005b_002elg_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.lg]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002elg_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002elg-1"></a>
<a class="index-entry-id" id="index-activating-ligatures-_0028lg_0029"></a>
<a class="index-entry-id" id="index-ligatures_002c-activating-_0028lg_0029"></a>
<a class="index-entry-id" id="index-ligatures-enabled-register-_0028_002elg_0029"></a>
<p>Switch the ligature mechanism on or off; if the parameter is non-zero or
missing, ligatures are enabled, otherwise disabled. Default is on. The
current ligature mode can be found in the read-only register <code class="code">.lg</code>
(set to 1 or 2 if ligatures are enabled, 0 otherwise).
</p>
<p>Setting the ligature mode to 2 enables the two-character ligatures
(fi, fl, and ff) and disables the three-character ligatures (ffi and
ffl).
</p></dd></dl>
<p><em class="dfn">Pairwise kerning</em> is another subtle typesetting mechanism that
modifies the distance between a glyph pair to improve readability. In
most cases (but not always) the distance is decreased.
Typewriter-like fonts and fonts for terminals where all glyphs have the
same width don’t use kerning.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_002ekern"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.kern</code></strong> <var class="def-var-arguments">[<span class="r"><i class="slanted">flag</i></span>]</var><a class="copiable-link" href='#index-_002ekern'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-kern"></a>
</dd><dt class="deffnx def-cmd-deffn" id="index-_005cn_005b_002ekern_005d"><span class="category-def">Register: </span><span><strong class="def-name"><code class="t">\n[.kern]</code></strong><a class="copiable-link" href='#index-_005cn_005b_002ekern_005d'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-_002ekern-1"></a>
<a class="index-entry-id" id="index-activating-kerning-_0028kern_0029"></a>
<a class="index-entry-id" id="index-kerning_002c-activating-_0028kern_0029"></a>
<a class="index-entry-id" id="index-kerning-enabled-register-_0028_002ekern_0029"></a>
<p>Switch kerning on or off. If the parameter is non-zero or missing,
enable pairwise kerning, otherwise disable it. The read-only register
<code class="code">.kern</code> is set to 1 if pairwise kerning is enabled,
0 otherwise.
</p>
<a class="index-entry-id" id="index-dummy-character-_0028_005c_0026_0029_002c-effect-on-kerning"></a>
<a class="index-entry-id" id="index-character_002c-dummy-_0028_005c_0026_0029_002c-effect-on-kerning"></a>
<p>If the font description file contains pairwise kerning information,
glyphs from that font are kerned. Kerning between two glyphs can be
inhibited by placing <code class="code">\&</code> between them: ‘<samp class="samp">V\&A</samp>’.
</p>
<p>See <a class="xref" href="Font-Description-File-Format.html">Font Description File Format</a>.
</p></dd></dl>
<a class="index-entry-id" id="index-track-kerning"></a>
<a class="index-entry-id" id="index-kerning_002c-track"></a>
<p><em class="dfn">Track kerning</em> expands or reduces the space between glyphs. This
can be handy, for example, if you need to squeeze a long word onto a
single line or spread some text to fill a narrow column. It must be
used with great care since it is usually considered bad typography if
the reader notices the effect.
</p>
<dl class="first-deffn">
<dt class="deffn" id="index-_002etkf"><span class="category-def">Request: </span><span><strong class="def-name"><code class="t">.tkf</code></strong> <var class="def-var-arguments">f s1 n1 s2 n2</var><a class="copiable-link" href='#index-_002etkf'> ¶</a></span></dt>
<dd><a class="index-entry-id" id="index-tkf"></a>
<a class="index-entry-id" id="index-activating-track-kerning-_0028tkf_0029"></a>
<a class="index-entry-id" id="index-track-kerning_002c-activating-_0028tkf_0029"></a>
<p>Enable track kerning for font <var class="var">f</var>. If the current font
is <var class="var">f</var> the width of every glyph is increased by an amount
between <var class="var">n1</var> and <var class="var">n2</var> (<var class="var">n1</var>, <var class="var">n2</var> can be negative); if
the current type size is less than or equal to <var class="var">s1</var> the width is
increased by <var class="var">n1</var>; if it is greater than or equal to <var class="var">s2</var> the
width is increased by <var class="var">n2</var>; if the type size is greater than or
equal to <var class="var">s1</var> and less than or equal to <var class="var">s2</var> the increase in
width is a linear function of the type size.
</p>
<p>The default scaling unit is ‘<samp class="samp">z</samp>’ for <var class="var">s1</var> and <var class="var">s2</var>, ‘<samp class="samp">p</samp>’
for <var class="var">n1</var> and <var class="var">n2</var>.
</p>
<p>The track kerning amount is added even to the rightmost glyph in a line;
for large values it is thus recommended to increase the line length by
the same amount to compensate.
</p></dd></dl>
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