Linelength or measure refers to the length of a line of text (caps, lowercase and numerals.
Tracking or letter-spacing refers to a consistent and uniform degree of increase (or sometimes decrease) of space between letters to affect overall density and texture in a line or block of text.
Kerning (less commonly mortising) is the process of adjusting the spacing between individual characters in a proportional font, usually to achieve a visually pleasing result.
Linelength significantly affects readability. The eye tires if a line requires more than 3 or 4 saccadic eye movements. Each movement is able to take in and recognise the meaning of around 3 words at once. This means lines should ideally not be more than 9-12 words long.
However optimum line length is also relative to each design, and often determined by other factors than readability alone. In this case other ways of increasing readibility should be used eg changing linespacing or contrast and careful consideration of typespace.
Tracking or letterspacing
The amount of letter-spacing in text can affect legibility.
- As reading with phonetic writing systems is based in part on context, and with unfamiliar words, on phonetic pronunciation, recognition of individual characters can be aided by slightly increased letter-spacing.
- Tight letter-spacing, particularly in small text sizes, can diminish legibility.
- The addition of minimal letter-spacing can often increase the legibility and readability.
Added whitespace around the characters allows the individual characters to emerge and be recognized more quickly. But addition of space to the point that individual letters become isolated rather than simply easily identifiable destroys legibility and readability.
The amount of letter-spacing can also affect how text is perceived and emotional response.
- Tight default letter-spacing, or negative letter-spacing can trigger a cultural association that tight letter-spacing is associated with advertising and therefore more subjective – the equivalent of a fast-talking car salesman.
- The increase of letter-spacing in text has a cultural association of a more objective typographic voice.
Until the advent of phototypesetting, the term “letterspacing” referred strictly to the adding of space between the individual letters of words set in metal type.
The amount of added spacing always had to be the same between each character, in increments of a minimum of ½ point. Fixed spaces include:
- em-space: the same width as the current point size
- en-space: half the current point size
- a hair space
- thin space
Letterspacing as such was expensive, involving the hand insertion of copper (½ pt.), brass (1 pt.), and printer’s “lead” (2 pt.) spaces between individual pieces of type or between matrices on linecasting machines such as the Ludlow Typograph and the Linotype. As such, it was studiously avoided by compositors, as adding nothing more than time to an already laborious task.The only exceptions were in advertising type or, in book work, in very short phrases in capitals or small capitals, to keep the phrases from being too visually black compared to the rest of the typographic composition.
Historically, personal computer based applications like Microsoft Word, Adobe Illustrator, QuarkXPress, Adobe InDesign, Adobe Photoshop, and Wordperfect, use differing, non-standard systems of adding or subtracting letter-spacing. What is common to most systems is that the default setting of letter-spacing or tracking is zero, using the widths (and kerning information) built into the font itself. Although digital type sets tighter on average than metal type, this is primarily a function of design decisions in the fonts, and the more ready availability of kerning, rather than any design choice inherent in the technology.
Letter-spacing adjustments are frequently used in news design. The speed with which pages must be built on deadline does not usually leave time to rewrite paragraphs that end in split words or that create orphans or widows. Letter-spacing is increased or decreased by modest (usually unnoticeable) amounts to fix these unattractive situations.
Kerning or mortising
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 a well-kerned font, the two-dimensional blank spaces between each pair of characters all have similar area.
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 (monospaced) fonts do not use kerning, since their characters always have the same spacing.
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. A corner was notched to a consistent height on one or both sides of a letter-piece. Such notched pieces were only set against one another, not against unnotched ones, which had straight sides. The corner allowed for a character’s features to reach into the area normally taken up by the next character. For example, the top bar of a T or the right diagonal stroke of the V could hang over the bottom left corner of an A. Having a consistently shaped corner cut out allowed for using fewer pieces of type to make up all possible kerning pairs. For example, a T and V piece with kerning on the right would match the same A piece with a matching kerning indention on the left.
An alternative is to have ligatures for common glyph combinations, such as the French L’, or the combinations ff, fi and ffi.
a kern in that sense could only bring letters closer together (negative spacing), though of course it was possible to add space between letters. With the arrival of digital fonts, it became much easier to kern many glyph combinations. Digital kerning can go in either direction – closer or further apart.
Kerning is usually applied to letter pairs as a number by which the default character spacing should be increased or decreased. Reducing the default character spacing 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 (full stop) 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 often use kerning are FA, LT, and LY. Increased character width is used mainly in conjunction with punctuation symbols (for example, the lower case letter f followed by right parenthesis or quotation mark) and accented letters.
Some typographic programs provide an autokerning feature. Autokerning simply takes into account a predefined list of common kerning pairs, and if the outlines of two consecutive glyphs are spaced too far apart, makes a kerning entry. Autokerning is especially useful for kerning multi-language fonts. However, it is rarely a sufficient alternative for manual kerning, as some characters may appear to an algorithmic comparison to be spaced very closely together, but to a human reader might appear to be spaced too far apart, especially when the only part of a glyph that is ‘too close’ is a diacritic sign.
The OpenType format permits group-based kerning, which facilitates kerning for fonts that have a large number of glyphs. Instead of specifying the kerning for ‘Va’ and ‘Vá’ separately, all diacritics with the base letter ‘a’ are placed into a group and the kerning between V and this group is specified. At the same time it is possible to add exceptions, e.g. for ‘Vā’, to the group. Group-based kerning is supported in nearly all modern office and desktop publishing applications.