title_body: Are there studies on whether reading in landscape or portrait mode is better on smartphone size screen? Are there studies on the whether it's better to read ebooks in landscape or portrait mode on typical smartphone screen? "Ebook" here implies that the text fills up most of the page with free reflow (e.g. not a PDF that has the same characters on each line in both modes), many pages read one after another; and the font size remains identical between portrait and landscape.
  
  Screen sizes I care about are of typical smartphones, e.g. around 4.5"-5.5", typically circa 16:9 or 16:10 factor.
  
  The measurements defining "better" can be (at least those I can think of) either reading speed, the content retention, eye tiredness, or mental tiredness.
upvoted_answer: Some where (I can't find it right now), I read that 72 characters per line was best for reading comprehension. When there was less than this the end of line moving your eye to the begging of the next line slowed you down, and when there was more than this following lines become more difficult, because you have to move your head which makes eye tracking more difficult.
  
  The study did not take into account dyslexia.
  
  So I guess it depends on your font size.