In typography, kerning is the process of adjusting the spacing between characters in a proportional
font, usually to achieve a visually pleasing result.
Kerning adjusts the space between individual letter forms, while tracking adjusts spacing
uniformly over a range of characters.
In a well-kerned font, the two-dimensional blank spaces between each pair of characters
all have a visually similar area.
The related term kern denotes a part of a type letter that overhangs the edge of the
type block.
Metal typesetting
The source of the word kern is from the French word carne, meaning "projecting angle, quill
of a pen".
The French term originated from the Latin cardo, cardinis, meaning "hinge".
In the days when all type was cast metal, parts of a sort that needed to overlap adjacent
letters simply hung off the edge of the sort slug.
The bit of metal that hung over the edge was called a kern.
At that time, the word "kerning" only referred to manufacturing the sorts with kerns, while
adjusting space between letters during compositing was called inter-spacing or letter spacing.
Because this method was not well-suited to some pairs of letters, ligatures were supplied
for those glyph combinations, such as the French L’, or the combinations ff, fi and
ffi.
Digital typography In metal typesetting, kerning was labour-intensive
and expensive because the matrices had to be physically modified.
It was therefore only employed on letter combinations which needed it the most, such as VA or AV.
With the arrival of digital fonts, it became much easier to kern many glyph combinations.
Kerning values In digital typography, kerning is usually
applied to letter pairs as a number by which the default character spacing should be increased
or decreased: a positive value for an increase, a negative value for a decrease.
The number is expressed in font units, one unit being a certain fraction of an em.
Different fonts may use different units, but common values are 1000 and 2048 units/em.
Thus, for 1000 units/em, a kerning value of 15 means an increase in character spacing
by 0.015 of the current type size.
Most kerning adjustments are negative, and negative adjustments are generally larger
than positive ones.
Adjustments for different pairs within a given font can range from a tiny 2 to over 100.
The adjustments for a given pair vary greatly from one font to another.
Negative kerning is widely used to fit capital letters such as T, V, W, and Y closer to some
other capital letters on either side, especially A, and to some lower case letters on the right
side, such as the combinations Ta, Te, and To.
It is also used to fit a period or a comma closer to these and to F and P, as well as
to the lower case letters r, v, w, and y.
Some other combinations that use negative kerning are FA, LT, and LY, and letters like
A, L, and h followed by a quotation mark.
Positive kerning is used mainly in conjunction with special characters and punctuation.
Depending on the font, some small positive kerning may also be required for accented
letters and for pairs like Bo, Dw, and TY.
The table below contains a few exemplifying kerning pairs and their values.
These values are based on 1000 units/em and the kerning pairs are ordered from the most
negative to the most positive kerning value.
The samples are taken from the kerning tables of the Minion Pro font.
In other fonts the kerning may be very different.
Which letters need to be kerned depends on which languages the font is to be used with.
Since some combinations of letters are not used in normal words in any language, kerning
these is not necessary.
Non-proportional fonts do not use kerning, since their characters always have the same
spacing.
Kerning tables In older font formats, such as Microsoft's
TrueType, the kerning values are specified in a simple kern table where each entry consists
of a pair of characters and their kerning value.
TrueType fonts typically have several hundred pairs, but some have more than a thousand.
Since OpenType is a superset of TrueType kern tables are still supported for TrueType fonts
packed as OpenType; however PostScript-based OpenType fonts don't have this option.
OpenType introduced a new, uniform way of specifying, among other things, kerning, via
the Glyph Positioning Table.
The more recent font releases by Adobe no longer have kern tables at all, but only specify
kerning via GPOS.
Since an OpenType font may include thousands of glyphs, and consequently a huge number
of pairs of characters that need kerning, OpenType fonts may have an elaborate system
of tables and subtables, designed to minimize the overall storage space.
The system is based on the concept of glyph classes: instead of a one-dimensional table
where each entry corresponds to a pair of characters, there are two-dimensional tables
where each entry corresponds to a pair of classes of glyphs.
A class includes several characters whose right-hand outline is identical for kerning
purposes, or several characters whose left-hand outline is identical.
All pairs of characters where the first one is from the first class and the second one
is from the second class will require the same kerning value, so this value needs to
be specified only once in the table.
The rows in the two-dimensional table correspond to first-character classes, and the columns
correspond to second-character classes.
The kerning value for a given pair of characters is found in the table at the intersection
of the classes to which they belong.
This system is very economical, but is necessarily limited.
For example, many of the classes may be quite small.
Also, a font with many types of glyphs may require several such tables.
Finally, many pairs remain that cannot be represented through classes.
For them, simpler, one-dimensional tables are provided: each table is for a particular
character that is the first in many pairs, and the entries contain the characters that
are the second in these pairs, together with the corresponding kerning values.
Here are a few examples of glyph classes in the Minion Pro font for the first character
in a kerning pair:,,,,,,,; and for the second character in a pair:,,,,,,.
A type of letters that lend themselves well to class-based kerning are those with diacritical
marks.
These letters can be added to the class of the base letter, and can stay together whether
they are the first or second character in a pair:,, etc.
A letter cannot be included in the class if its kerning is different from the others in
certain pairs.
Most modern office and desktop publishing systems support OpenType features, and hence
class-based kerning.
Automatic kerning
Automatic kerning refers to the kerning applied automatically by a program, as opposed to
no kerning at all, or the kerning applied manually by the user.
There are two types of automatic kerning, metric and optical.
With metric kerning, the program uses directly the values found in the kerning table(s) included
in the font file.
Most systems with typographic features today provide this type of kerning.
Optical kerning, on the other hand, is available only in the more advanced systems.
With optical kerning, the program uses an algorithm to calculate, from their outlines,
the optimal spacing for each pair of consecutive characters.
With both types of automatic kerning, the system usually permits the user to specify
a minimum font size for applying kerning, if the user feels that kerning is unnecessary
for smaller font sizes.
With metric kerning, in a text that uses several fonts, the program must decide which kerning
table to use when two consecutive characters belong to different fonts: the table from
the font of the first character, or the second one, or to avoid kerning altogether.
In this case optical kerning is preferable.
A common situation occurs when italic text ends with a roman symbol and the last letter's
slant clashes with the symbol.
Manual kerning, available in some systems, permits the user to override the automatic
kerning and to apply any kerning value directly to a pair of characters in a particular place
in the text.
When not available, this feature can be simulated by using, for those two characters, the function
that modifies the space between characters in a block of text.
When employed by a skilled person, manual kerning will usually give better results than
optical kerning; for example, some characters that may appear to an algorithmic comparison
to be spaced very closely together may appear to a human reader too far apart, especially
when the only element of a glyph that is “too close” is a diacritic mark.
Manual kerning may even be better than the metric kerning built into the kerning table
by the font's designer, since these tables often have errors or omissions, or the difference
may simply be a matter of personal preference.
Contextual kerning Contextual kerning refers to positional adjustment
that depends on more than two consecutive glyphs.
For example, the spacing of a certain glyph may depend not only on the preceding glyph
but also on the one following it.
Although rarely implemented in ordinary documents, contextual kerning is a concern in quality
typography.
An example of a situation that demands contextual kerning in the Minion Pro font is the sequence
of three characters f.”, as is often found at the end of a quotation.
Using the font's kerning tables, the quotation mark is too close to the f, although without
the period between them their spacing is adequate.
The period, in other words, reduces their spacing instead of increasing it.
The explanation is this: Without the period, their kerning is a positive 121.
The period's width is 228, but the kerning between f and the period is −5, and between
the period and the quotation mark −138.
The total is a positive 85, as opposed to the original 121: a net loss of 36 units,
which explains why the quotation mark is now closer to the f.
Contextual kerning would recognize the sequence of three characters, and would increase one
or both inter-character spaces.
A similar problem exists with the letters F, P, T, V, W, and Y; with comma rather than
period; or with single rather than double quotation mark.
Contextual kerning is supported by the OpenType font format, but few font designers implement
it, and probably no desktop publishing systems can currently use it.
When important, the solution for a user is to employ manual kerning instead.
Kerning tools Font editors allow the user to modify the
properties of a font, including its kerning table.
They accomplish this by modifying the table found in the actual font file.
The user can change the kerning value in existing pairs, or add new pairs.
A few desktop publishing systems allow the user to change or add kerning pairs without
modifying the font file itself.
The system merely applies to the user's document the new kerning values, instead of the values
found in the font file.
Whether modifying the font file with a font editor or overriding it in a particular system,
the user is limited to the existing kerning features.
Thus, if one needs features like optical kerning, or contextual kerning, or kerning a pair of
characters that belong to different fonts, and if the system lacks these features, other
means must be employed.
Some desktop publishing systems permit developers to create plug-ins, and this capability has
also been used for kerning.
In general, these plug-ins permit the user to apply a kerning change automatically to
a certain character pair throughout an entire document, instead of applying it by searching
manually for those pairs.
So far only basic kerning features have been implemented through plug-ins, and it is unclear
whether the more advanced features can be effectively implemented in this manner.
Instead of adding functionality to a desktop publishing system, a different approach is
to export the document and process it outside that system.
Any kerning features can then be applied to the document using tools ranging from ordinary
text editors to programs specially developed for this task.
The modified document is then imported back into the desktop publishing system.
Many systems permit this operation, either by converting the document with import and
export functions, or by making their internal document format an open standard.
The benefit of this approach is that some complex typesetting functions that may be
hard to implement through plug-ins may be relatively easy to implement through separate
tools.
An example of this approach is Software and Mind, a 944-page book produced using Adobe
Systems' FrameMaker desktop publishing system.
In an attempt to achieve “perfect” typesetting, the entire book was created in MIF and then
imported into FrameMaker.
MIF can represent entire FrameMaker documents in a relatively simple, ASCII-based format,
which can be processed with any software capable of handling text.
Thus, improved kerning was implemented through MIF instructions generated by specially developed
software, and the final result was imported into FrameMaker.
This resulted in a document that would have been all but impossible to create in FrameMaker
manually or through plug-ins.
Kerning in browsers The CSS property text-rendering: optimizeLegibility;
enables kerning in Firefox, Chrome, Safari, Opera, and the Android Browser.
There is also a proposed CSS3 property font-kerning, but it is only supported in Firefox, Chrome
and Opera and in Internet Explorer starting at version 10.
The CSS3 draft suggests that kerning should always be enabled for OpenType fonts.
Kerning perception
The human perception of kerning can vary with the intraword and interword spacing during
reading.
A visually pleasing result, even with no "kerning control", can be achieved with some control
of the space between letters.
For instance, on webpages with CSS1, a standard dating back to 1996, the letter-spacing property
offers some control for lost or enhance "kerning perception" — kerning can be simulated with
non-uniform spacing between letters.
The standard CSS3 includes the font-kerning property, which allows a complete control
of kerning.
For rather more technical reasons some have proposed to replace OpenType-style GPOS kerning
with spacer glyphs using OpenType's Glyph Substitution Table.
See also Letter-spacing
Typographic ligature Word spacing
References
External links http:www.printmag.comthe-kerning-game/
http:blog.typekit.com02kerning-on-the-web/ The dictionary definition of kerning at Wiktionary