Typography Tip #3 recommends setting linespacing in Word to a multiple of 1.2. This has the effect of making each line of 10 point text 12 points tall. Is this a good recommendation?

Yes.

Miles Tinker ran a huge research program investigating typographic variables from the 1920s to the 1950s. Linespacing is one of many variables he thoroughly investigated. He collected data with multiple sizes of text and multiple line lengths. His standard methodology measured people’s reading speed while simultaneously carrying out a comprehension task that ensured that the text was fully understood. This ensured that he was only measuring reading speed differences and not comprehension differences.

When examining reading speed with 10 point text, Tinker found little difference between reading speed with text set solid and adding 1 point of linespacing (multiple of 1.1). A statistically reliable speed advantage was found by adding 2 points of linespacing (multiple of 1.2). And adding more linespacing past 2 points did not further improve reading speed. 2 points of linespacing appears to be the critical amount of extra space needed to separate lines, as 2 points was also optimal for both 8 and 12 point text sizes.

Linespacing with 10 point text:

Amount of Additional Linespacing	Reading Speed Difference in Percent
Set Solid (control)	0.0
1 point	-1.0
2 point	+5.2
4 point	+2.8

 

Bonus: In the comments for Typography Tip #3, Adam Twardoch asserts that the line length effects the amount of needed linespacing. Tinker’s data does not back up this assertion. This table shows that 2 points of linespacing performed the best at each line width tested.

Linespacing with 10 point text. Reading speed difference in percent compared to 19 pica line (3.2 inch) with 2 points of additional linespacing:

Line Width	Set Solid	1 point linespacing	2 point linespacing	4 point linespacing
9 pica (1.5 inch)	-9.3	-6.0	-5.3	-7.1
14 pica (2.3 inch)	-4.5	-0.6	-0.3	-1.7
19 pica (3.2 inch)	-5.0	-5.1	0.0 (control)	-2.0
31 pica (5.2 inch)	-3.7	-3.8	-2.4	-3.6
43 pica (7.2 inch)	-9.1	-9.0	-5.9	-8.8

Cheers, Kevin Larson

Miles A. Tinker, Legibility of Print, Iowa State University Press, 1963.