In the year 1898 the English typefounders, as a body, adopted a system (which had been in use in America since 1878) of casting their types to a certain fixed standard. That standard was the American pica, 83 of which equalled 35 centimetres. The pica, which measured 4.21mm, was divided into 12 equal parts called 'points', which makes the size of a point approximately 0.35 mm.

6 picas equal approximately 25 mm;
there are 12 points to 1 pica

The different bodies or sizes of types are expressed by the number of points, a number which also shows the relative proportions to each other, as 6-pt, 8-pt, 10-pt, 12-pt, etc., and all types, rules, borders, etc., are made on this standard unit of measurement.

In Printing Shops the pica, now called twelve points, is still retained as the standard unit of measurement for the length of typesetting, rules, etc., and for the various width and depths of book pages and other matter set up by the Compositor. This is because it would be inconvenient to translate long lengths into points. It is easier to refer to pages widths and depths as 24 picas, 30 picas, 45 picas, etc., than to multiply these measures by 12 and call them 288 points, 360 points, 540 points, respectively. Metric measurements are used for sizes of paper and cardboard.

The Em
Where a constant-width blank space is required, you use spaces of certain fixed increments. This is when the other traditional unit of typographical measurement, known as the 'em', i.e. the letter m, is used. The em has a maximum relative unit value, so, in a 54-unit system, the em is 54 units ... and so on for 18- and 36-unit systems.

It must be remembered that any measurement given in ems is not by itself a fixed size, but is meaningful only in relation to the size of type in use; 'one em indent' in 8-pt type is 8 points, but in 10-pt is 10 points. In most type families the em space is designed as a square of the point size. Therefore, units are relative, the larger the point size being used, the larger each of the units will be, but the number of units will not change.

Since the point system is interrelated, the em is commonly used as a substitute for the pica but care must be taken when specifying what it is you want. The instruction '24 ems wide' is technically meaningless if no type size is given; but '24 pica ems wide' is definitive. Another danger lies in the use of indents. A 3 pica indent would always equal 36 points but a 3-em indent will vary depending on the size of type in use.

The indents for this setting in 9-pt type are
different when ems and picas are used

In most type families the em space is designed as a square of the point size, another fixed space associated with the em is the 'en' which is proportionately half the width of the 'em' and the 'thin' space which is either one-third or one-quarter of the width of the em.

In these cases, while always being the same height as the point size, the widths of the fixed spaces might vary according to the style of the type.

Fixed spaces were originally based
on an 18 unit maximum value.

(Compositors in the nineteenth century were paid by the 1000 ens per hour that they could produce. Today, the en is still used by some typesetters for costing purposes but for the most part, it is restricted to certain copyfitting calculations.)

Set Width
Every letter and space in some fonts of type has a definite width or 'set' (thickness from side-to-side), which is a multiple of a point or even fraction thereof, so that point-set types are as exact in their widths as in their depth. The typefounder establishes the set of each character in a font at the first casting, and this width is maintained in all subsequent castings. It should be remembered that the set only effects the letters and not the word spacing.

These letters were all cast on the same size body but have a different width or 'set'

For the vast majority of typographical layouts, knowledge of set is of little practical value. However, there is the occasional job where it can make all the difference between whether or not a keyboard operator can make a designer's layout work.

Tracking controls the set-width of type

The standard method used to vary the set of type is known as 'tracking', or minus-setting, because the increments are actually subtracted from the number of units allotted to each letter. Copy can be typeset using Normal, Tight, and Very Tight.

Type Size
The basic unit of measurement in typography is the point. All other dimensions and terms used in printing derive from this one unique measurement. The point is used to describe the differences in size between type faces, line spacing, and other elements of composition, but also leads to great confusion. In North America and Britain the point is approximately .351mm and is called the Pica point. In Europe the point is a little bigger (.376mm) and is called the Didot point.

In both systems, points have always been used to describe the length of one metal chunk of type. A 72-pt H, in metal type, is a character cast onto the top of a metal block; the block carries the letter through all the printing operations and the block's top surface is itself exactly 72 points (or .918") in height.

The actual impression or image height of the H, when printed on paper, will be smaller than the overall size of the metal. Traditionally, the point size of type refers to a specific dimension of the metal and not to the image height. This discrepancy is necessary because of the ascenders and descenders.

Point size is only the expression used for the distance between ascender and descender and as such cannot describe the proportional relationship of any type face's x-height to its ascenders and descenders. If one chooses two different faces, such as Helvetica and Garamond, they will appear on the page very differently.

Helvetica (left) and Times (right)
typeset in the same size!

Since it is the x-height of the type face which most directly influences its appearance on the page, a 36-pt face with short ascenders and descenders will have a much greater visual impact. Both of the faces above are 36-pt, yet notice how much larger the Helvetica seems by virtue of its x-height. This incongruity is further demonstrated below when using a script face.

Even with the problems as illustrated, the usefulness of the point in describing type size and the like cannot be denied. When you are discussing very small type, it is much more preferable to deal in round numbers (rather than fractions of inches or decimal places with millimeters).