The distance between the words of a line of text is called word spacing. To enable a reader to easily distinguish between individual words, the distance between the last letter of one word and the first letter of the adjacent word needs to be significantly larger than the distance between adjacent letters within one word. However, leaving holes in a body of text because of wide spacing looks improper and is uneconomical. Typographers and researchers seem to agree that the optimal distance between two words is 25 percent of the typesize (Hartley, 1978). For all practical purposes, this can be achieved by treating the space as if it were an ‘i’. Thus, if the size of type is 12 points, two words should ideally be separated by 3 points in distance. When regular letter spacing is used, this allows for easy distinction between words. However, Søgren’s (1995) ‘m’-rule for letter spacing, introduced earlier, is not compatible with the ‘i’-rule just discussed; the two proposed distances are about equal. Looking at figure 12 and considering the proven merits of condensed letter spacing, it seems advisable to hold on to the ‘i’-rule and to set letters within one word more closely.

Figure 12:
The ‘i’-rule of Thumb: the first, regularly spaced sentence is equally long as the second, in which the spaces have been replaced by the letter ‘i’

No research has been published on conventional word spacing as an isolated variable. All statements about optimal spacing dimensions, like those presented here, seem to be based on practical experience in typography. However, research has been reported about the effect of justification on word spacing. When text will be justified, it is rather useless for the designer to determine word spacing in the way discussed above. The problems with word spacing that result will be discussed in the next paragraph.