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Readability: Justified vs. Ragged

Derek Powazek writes, in his Note to Apple: Justified text decreases readability on Flickr:

Safari 5's new "Reader" feature makes my site less readable by justifying the text. Flush-left text (aka ragged-right) is demonstrably more readable, especially when the rendering engine doesn't know how to hyphenate. You never, ever justify text without hyphenation.

Justified text was invented to define columns of text in multi-column layouts (like newspapers). Why they chose to justify the text in a 1-coumn view that's supposed to be more readable is beyond me.

The point about poor hyphenation must be right, but what about the claim that Flush-left text ... is demonstrably more readable? This is widely claimed, but the evidence isn't overwhelming.

First, a point of terminology: flush left and ragged right are actually independent constraints on paragraph layout, and all of the four combinations of either flush or ragged left with either flush or ragged right have their place. Flush right generally means that the right edges of the all the rightmost letters line up —although note that this will not be exactly true if text layout uses microtypography— while ragged right means allows the horizontal distance between the rights edges to vary. Justified text is text that is both flush left and flush right, and for our purposes here, we can simplify and consider only flush left text.

The key point, then, is that ragged right is consistent with using the same width space between words, whilst justified text must allow the interword space to vary.

In an influential article, Williams (2000, p394) recommends the use of [s]et type intended for extended reading flush left, and ragged right because (p390) [n]on-uniform spacing between words decreases reading speed by as much as 11 percent (Trollip and Sales 1986).

However, I believe that such an increase in reading speed depends on lines being relatively long, where the speed up arises from the better rhythm the eye can achieve as it moves between eye fixations. As lines become shorter, the proportion of time the eye spends moving from the end of one line to the beginning of the next becomes larger. This should be faster with justified text for two reasons:

  1. Short lines tend to happen because of either the existence of inset items such as photographs, or multi-column text. Good page layout demands more space between the right edge of a text column and what is to its right for ragged right text than for justified text, because with justified text it is easier for the reader to accurately grasp the shape of the text block (cf. Williams 2000, p387, on complexity). This difference in needed margin is quite significant, and so when there is little width, justified text will have a lower proportion of expensive line jumps to regular, in-line eye movements.

  2. The line jumps are regular with justified text, since the right edge of each line is a constant horizontal distance to the left edge of the next line. Thus, with short lines, there is a better possibility of generating eye-fixation rhythm from line to line with justified text than with ragged.

So, as the line becomes shorter, the case for faster readability of ragged text becomes weaker. With multicolumn page layouts, it may be the case that justified text is generally more readable; certainly, the practice generally is to use justified text with multicolumn layouts. To answer the question: which should I use, I recommend the advice of Newsletter Fillers.

References

Trollip, S. R., and G. Sales, 1986. "Readability of computer-generated fill-justified text." Human Factors 28:159-163.

Williams, T. R., 2000. Guidelines for Designing and Evaluating the Display of Information on the Web. In Technical Communication 47(3):383-396.