Classically-trained designers, and really every professional designer, should know the old adage that long line lengths can have a counterproductive impact on readability. The trusty copy of Bringhurst's authoritative The Elements of Typographic Style makes this rule fairly cut-and-dry:

Anything from 45 to 75 characters is widely-regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size. The 66-character line (counting both letters and spaces) is widely regarded as ideal.

So naturally, when designers work on the web, they're keeping this rule in mind. Consequently, it's become a best practice on the web to keep line lengths below 75 characters, and this best practice has been the source of dissent against movements for things like variable-width (fluid) layouts and the like.

But research doesn't support this claim, at least on the web. Instead, users are able to read significantly longer line lengths on the web, and it actually increases efficiency and comprehension.

The Research

The first research I could find relating to this topic went all the way back to 2005. The study,The Effects of Line Length on Reading Online News, looked at how well college students read news on the screen at different line lengths. They tested line lengths of 35, 55, 75, and 95 characters. The study came to two important conclusions:

Reading speed was highest at 95cpl, and lowest at 35cpl on screen.

Reading efficiency was again highest at 95cpl.

They found that line length no impact on comprehension on the screen

These days, it would be a bit nicer to have a wider array of line lengths, perhaps going up to 115 or 135cpl, but this is a useful study.

Guesswork

In a very brief and informal search, I came across a healthy number of sites that had line lengths around or above 100 characters that seemed quite readable, including my own blog. So what makes the screen significantly different from print that would impact comprehension in this way?

A few guesses, with absolutely no basis in research, the third being my favorite:

We've been conditioned to longer lengths by websites that have tended to stretch the top end of this limit, as opposed to newspapers, which tend to stretch the lower end.

The active lighting of a computer display is more conducive to longer line lengths than passive paper.

Perhaps line lengths that are out of proportion with their medium are more difficult to read. Print tends to have vertical layouts, which would naturally correspond to shorter lines, while we tend to use widescreen monitors these days. Perhaps the widescreen monitors are more conducive to the readability of a proportionally-longer line length.

If the range of line lengths had been wider in the study, and if we had an updated study since the widescreen monitors had become popular, we could really see what might be impacting the difference.

Best Practices

So should you really be limiting your line length to 75 characters? This research suggests you shouldn't. Users will be perfectly fine reading longer columns of text.

What is the new standard? Tough to say, but 100cpl seems to be within the range of feasibility. There may be a good opportunity for some new and more thorough research in this area that could offer some valuable new insight.