Figure 18:
Leading: lines set solid, with line spacing 1, and with line spacing 1.5 > the red square is the area which used to be a seperate piece of type on the traditional printing presses - this is the same for all lines of text and its height is the corps size, or body height - leading is more than word spacing in versions 2 and 3

Paterson and Tinker (1931) compared 0-, 1-,2-, and 4-point leading in combination with 10-point type and 19 pica line width and found that reading speed is highest when 2-point leading was used. From the series of experiments that followed their research, a rule of thumb emerged. For any size of type, the leading that warrants optimal reading conditions is 125 percent of the typesize (Hartley, 1978). Thus, given a typesize of 12 points, two lines of text would ideally be separated by 15 points in distance (3-point leading). Note that word spacing should appear to be narrower than leading. If not, says Søgren (1995), the evenly spaced look of the text is broken up by ‘rivers’ that appear to run through it vertically.

Expressing leading in terms of points is common in the printing practice, but confusion may set in for frequent users of typewriters or computer word processors. These devices offer the possibility to space lines at 1, 1.5, or 2 times the regular spacing. Regular spacing corresponds with the setting regularly chosen for the particular typeface in which the text is printed (not solid setting), while 1.5 times spacing refers to a setting in which the width of the space between lines is twice as high as in regular spacing. To how many points of leading this corresponds with depends on the typesize.

When line spacing is increased, it becomes more probable that the reader skips to the correct next line after having arrived at the end of the reading line. When lines of text are very long, it is especially difficult to end up at the correct next line. In that case, much leading should be chosen. However, the other extreme is just as undesirable. When the lines are very much separated, it will take longer to get to the next line, apart from the fact that it is uneconomical.

There is another way in which leading could affect reading comfort. "The successive lines of printed text form a pattern of ‘stripes’". These striped properties of text cause ‘eye- strain’ and sometimes even epileptic seizures. In a study of this effect, Wilkins and Nimmo Smith (1987) found that varying line spacing affects subjects’ judgments of text clarity. In one experiment, subjects’ responses to a text set in 9-point type were compared with responses to the same text set in 11-point type, both with 1 point of leading and controlling for lighting conditions, reading distance and exposure time. They found that relatively little leading prompted more reports of reading discomfort and that the size of disturbing illusory patterns observed within the text block was greater with less leading. Based on their findings, Wilkins and Nimmo Smith suggest that line spacing should be increased slightly over the conventional measure in order to minimize disturbing effects on reading comfort. They argue that when economy requires narrower spacing, preference should be given to narrowing letter spacing and word spacing over maintaining conventional line-spacing. They claim that reading comfort is affected least in this way. The proposal to increase leading seems to be supported by results from research by Hartley, Young and Burnhill (1975). They found that typesize is judged to be smaller when lines are set solid than in the case that there is more leading. It may be that reading comfort increases with more leading as a result of this perceived increase in typesize.