Line length revisited: following the research

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Mary Dyson produces nitty gritty research on the long-accepted notion that shorter line lengths are more legible than longer ones. The study finds that shorter lines do not necessarily lead to faster reading. If you’re looking for a definitive answer to use in your next design review debate, though, no dice. The big finding is that long lines don’t slow things down as much as previously thought, not that they’re better or worse.

But there’s so much more meat in here that I found much more interesting, mostly because I’m largely ignorant on the topic and gained a dollop of context on writing, legibility, and behavior.

Things like…

There’s a term for transitioning between lines of text

It’s return sweeps. You know, like your eye hits the Return key at the end of the line and sweeps to the start of the next line. Then, there are undershoots. The idea is that eyes may not sweep to the exact start of the next line, instead stopping a bit short.

Showing four muted lines of test with jump arrows across each line taking the eye to the end of the line, followed by dashed arrow leading to the next line. Red arrows highlight where the eye could undershoot a new line and miss content.

Those little rapid eye movements between words and phrases? Those are called saccades. I had to look that up.

The impact of undershoots is what’s being challenged

The previous research we’ve relied on for years comes from 1940(!), a time when we obviously were more concerned with paper pages than bright digital displays. Longer lines, it said, increased the likelihood that eyes undershoot during a return sweep, and undershooting results in a regression that results in a 130ms to 250ms delay where the brain needs to get its bearings. The report refers to that as undersweep-fixation.

We can still process words during undersweep-fixation

This report cites a 2019 study that tried to correct undershoots by bolding the first word at the start of each new line, sort of like an anchor that naturally draws the eye closer to the left margin.

The 2019 study did find that the bolded words did decrease undershot return sweeps But despite that, reading speed did not improve. That’s the impetus for challenging the long-held assumption that shorter is better.

Mary explains further:

In seeking to reconcile why longer line lengths may not slow down reading on screen but do so when reading print, I outlined some differences, e.g. visual angle, time spent scrolling. But although physical differences between reading from screen and reading print still exist, we now have direct evidence to explain why longer lines were read at least as fast as shorter lines. Readers can process words during the brief fixation when undershooting the start of the new line. This saves time in subsequent processing. Now we might also acknowledge that there is greater consistency between the range of optimal line lengths for print and screen.

Where does this leave us today?

Well, nowhere closer to a clear answer we can use in our day-to-day work. But it’s good to dust off our collection of design and copywriting best practices and know that line length is less of a constraint than perhaps it has been.

Again, none of this tells us whether long or short lines are better. Mary ends her report by saying she cannot definitely recommend using longer lines of text because there are clear because there are still some factors at play, including:

  • Shorter lines are more effective for people with dyslexia.
  • More questions about return sweeps and undershooting need to be answered.
  • Several other studies indicate that users prefer shorter lines and judge longer lines as more difficult to read.

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