At the simplest level, readability is about not getting between the reader and the content. According to Robert Bringhurst, "Typography with anything to say ... aspires to a kind of statuesque transparency." While magazine and advertising design is often about display typefaces, novelty and high impact, book design is much more self-effacing. Book designers aim for an intelligent understanding of the content they are typesetting, and type selection that aids that content.
There are a few rough rules of thumb for creating readable text. Serifed typefaces are generally easier to read than unserifed faces. Ornate, fussy typefaces should be reserved for headings/display type only. Generous interline spacing (120% of type height is often cited as an ideal) makes type easier to read, but only up to a point. 'Rivers' of white space running vertically through a poorly set body of text make it harder to read. Judicious hyphenation gives text a more even 'tone', but excessive hyphenation is distracting. Line lengths of approximately 10-15 words are ideal. A type size of between 10 and 12 points will work for most readers, ranging up to 16pts for those with some vision impairment. Narrow margins leave no space for a reader's hands, and paper thickness and colour also play a role in enhancing readability, not to mention lighting conditions. On monitors, screen contrast and brightness, refresh rates, type size and distance from the screen are factors.
Once the key factors are satisfied, fine-tuning readability is sometimes more a matter of aesthetics than any strict metric. Some typefaces just 'feel' better with a certain kind of content, and it is difficult to spell out the exact reason. Taste is a notoriously difficult concept to explain.
Professional level typesetting packages such as Quark or InDesign are much better at setting blocks of text than word processing packages. That said, observing the basic rules of readability will always yield a better result, whatever the package. Every word processing program gives its users some access to type controls, both at a character and a paragraph level. The typesetting program InDesign calculates the placement of words and hyphenation on a whole of paragraph basis, attempting to create an even type 'tone'.
The apparently simple act of reading is anything but. Letters are human constructions with a complex and conditional history. They are not necessarily optimal, and are subject to continuous reinvention, for aesthetic as well as functional purposes. Changes in printing technology and the advent of computers and the Internet have all precipitated waves of type innovation. Type designers spend a great deal of time designing letter forms, harmonising those forms through a whole family of weights and styles, then setting every possible combination of kerning pairs, ligatures, special characters and letters from other alphabets. All through this exacting process they exercise their informed judgement, and their knowledge of related and historical typefaces, and current developments in the field. It is very far from being a science, as precise as their measurements might be. There are so many variables in setting type for readability that there are probably infinite variations that will both satisfy the basic demands of readability and those of proportions and aesthetics. There will never be a utopian 'perfect' typeface, as one could always posit an improvement, or a circumstance might arise that demands a different approach.
Summary of Points to consider:
- type size
- type colour
- type clarity and contrast
- the ratio of the x-height to the overall letter height
- line length
- average word length
- frequency of hyphenation
- justified or set ragged left
- number of and space between columns
- leading (interline spacing)
- paper colour and thickness
- paper dimensions
Paying some attention to at least some of these parameters is bound to make a body of text much more readable. Bucking the tyranny of Arial, Times New Roman, document templates and unnecessary layout embellishment can be a rewarding path to take.
At a much more complex level, readability collides with psychology and neuro-anatomy. Scientists are interested in the way we read, whether a letter at a time, in clusters of letters, words, whole sentences or skimming whole paragraphs or pages. They look at culturally specific aspects of reading, and universal issues of cognition and meaning. Those apparently quotidian pages of text are moments away from being consumed and comprehended by the biggest mystery of all: consciousness. They are a way of one mind accessing the contents of another mind. That's one of the reasons that type design and typesetting is so endlessly interesting.