When people read, their eyes jump across a line of text, pausing momentarily to take in groups of three or four words. Studies have shown that readers can make only three or four of these jumps (or saccades) per line before reading becomes tiring.
Too long lines with too many words also make it harder for the eyes to find the correct spot when they sweep back to the left to pick up the next line of text. To maintain readability, it is imperative to use moderate line lengths within the range of 40–70 characters.
Do not set normally sized body text in a single column across a 8.5 by 11 inch page. If you do that, the result will greatly exceed 70 characters, reading efficiency will be significantly reduced, and your readers’ attention will be easily lost.
Two column layouts are the best solution for achieving optimal body text column widths on American letter size paper. If that is not practical and using only one column is necessary, then be sure to use large side margins in order to bring the line length as close as possible to 70 characters or less.
For body text, the optimized amount of leading (i.e., the space between lines of type in a paragraph) depends on the characteristics of the typeface in use. Common body text sizes are 9, 10, 11, or 12 points, but the same point size may look relatively large in one typeface and noticeably smaller in another.
The optimized amount of body text leading is generally between 1 and 4 points larger than the type size and is determined by testing to make sure that a full column of type doesn’t look too dense or heavy on the one hand or too spaced out on the other.
As the reader’s eye moves from left to right and then sweeps back to the left to find the next line of text, reading speed and accuracy are impeded by either too much or too little leading.
Once the optimized amount of leading has been determined for the body text typeface in use, it should be maintained throughout the document. Do not arbitrarily change the leading of a section of body text in order to force the text to fill out a fixed column height.
When reading a column of body text, the eyes start at the upper left corner and move down in a Z pattern, sweeping from left to right and back. Each rhythmic sweep to the left returns to the axis of orientation (the vertical line formed by the left side of the column) and the eye drops down to the next horizontal line of text (as if pulled by gravity).
If the placement of images such as charts and photos breaks the axis of orientation or forces the reader to work against reading gravity, reading rhythm will be broken, and concentration and comprehension will plummet.
To maintain readability, do not let images break into the middle of text columns. With few exceptions, the last line of text before an image should be the ending of a sentence or a paragraph. Wrapping text around an image or floating an image within a text block are techniques that should be used sparingly. If you do use them, the image should always break into the text from the right side, so that it does not interrupt the axis of orientation.
Type set in all capital letters is significantly harder to read than mixed case type (i.e., type set in lower case letters with capitals used only for the first letters of proper nouns, sentences, and headings).
Efficient reading depends partly on the eye being able to recognize the shapes of letters and words. We recognize letters by the shapes of their upper halves. That is easy to do with lower case and mixed case, because the shapes are distinctive and framed by the varying white space around them.
However, when lines of type are set in all upper case, the eye is presented with the look of a solid rectangle, shape recognition becomes a harder task, and reading speed is slowed by 10% or more.
It’s best to reserve the use of all capitals for small phrases and words that are just functioning as visual signposts (like the phrase NUMBER 4 in the headline of this section).
Studies have overwhelmingly shown that black type on a white background is easier to read than white type on a black background. Reading reversed-out text causes fatigue, and this effect applies to any light-type/dark-background combination.
It may seem eye-catching to style features like sidebars, callouts, and pull quotes by superimposing white type on a colored box, but the greater effort required to read reversed-out type may cause many readers to skip past the box without reading it at all.
A better feature styling solution is using a light tint of a color (10% or less) for the background, and superimposing black type with a medium or bold weight (slightly higher than the book weight type used for normal black body text). The heavier weight helps by thickening the lines of the characters, and you should always use this technique if you do decide to set reversed-out type.
Another approach is to restrict your use of full-color background boxes to blocks of display type, where the large size of the reversed-out type can help to mitigate the contrast problem.