In printing, all measures of typesize and spacing are expressed in points. Several kinds of points are used, according to the Didot-system, the Pica-system, or the IBM-Pica-system. One point equals 0.37606 mm, 0.35146 mm, or 0.35277 mm respectively. Typesize, also called corps-size, is based on the techniques once used in the metal printing presses and has lost its true meaning since. Corps-size includes space for ascenders and descenders and extra space on top and bottom, which varies between different typeface designs. Thus, the characters of one typeface set in 12-point size may give the impression of being larger than those of another typeface set in 12-points. Alternative measures of size are x-height, capital height, and total letter height (without extra space), but none of these is supported by a system (figure 1). The ratio of the height of ascenders and descenders to x-height is different for each typeface. Therefore, corps-size can not be calculated from x-height or the other way around. It is important, however, to keep the differences in definition in mind. This fact is illustrated by Poulton’s (1972) comparison of three typefaces with regard to legibility: Times, Perpetua, and Univers. In this study, Univers was found to be less legible than both other typefaces when all were printed in the same number of points. When the x- heights of all three were equated, however, no such difference was found. Thus, corps size is not a good indication of perceived size. Nonetheless, corps size is still used as the standard measure.

There are few absolute standards for typesize. Each application has a different set of requirements because of differences in reading distance, levels of illumination, and need for conspicuity. Increasing or decreasing the reading distance should be accompanied by a change in size and stroke width that keeps the visual angle constant in order to maintain the same levels of legibility and readability. The visual angle is based on reading distance and character height and is expressed in minutes or seconds of arc (figure 9). Table 2 contains recommended letter heights for various ratios of stroke-width -to- height at various distances and provides a set of formulas with which a suitable letter height can be calculated (Sanders and McCormick, 1993). In short, 22 to 27 minutes of arc is the convention for book reading and can generally be applied fairly safely to various other applications, assuming similar conditions such as normal levels of illumination.

Figure 9:
Calculation of the Visual Angle (after Sanders and McCormick, 1993)
d = distance between the eye and the character
h = character height (total letter height)
va = visual angle (in minutes)
va = 3438 x h/d

Nonetheless, caution is needed in the use of visual angle as an indicator of size, since the fact remains that some typefaces have to be set in 12 points to achieve the same effect of legibility and perceived size as is done with other typefaces set in 11 points. Thus, although we will continue by expressing typesize in points, it should be kept in mind that the sizes given all pertain to the ‘average’ typeface. For close up reading at a distance of 35 cm, such as in books, sizes between 9 and 11 points are predominantly used. Much research has been done to determine the optimal size, especially for application to books. In one of the first experimental studies of size, Paterson and Tinker (1929) concluded that 10-point type was read faster than type in other sizes. Subsequent research seems to agree on this. Typographer Søgren (1995) states that, in book print, 10- point type is best used for the text body, whereas 8-point type is most suitable for so called consultation text. At a reading distance of 75 cm, which could be expected when reading posters or signs, Giddings (1972) found that a character height of little less than 4 mm results in optimal reading performance (reading speed). For the commonly used typeface Times New Roman, this character height corresponds with a typesize of 15 points.

Table 2:
Recommended Letter Height (in centimeters) for various stroke wodth-to-height ratios at various distances (assuming a Snellen acuity score of 20/40); adapted from Sanders and McCormick, 1993

	Distance
Stroke width- to-height ratio	70 cm	3 m	6 m	30.5 m	305 m
1 : 6	0.25	1.06	2.12	10.60	106.0
1 : 8	0.33	1.41	2.83	14.15	141.5
1 : 10	0.41	1.77	3.54	17.68	176.8