These are picas:
and this is a foot , or at least a part of one.
The pica and the foot have something in common. They both divide into 12 useful parts. In the case of the foot, these are inches. Inches are the units used in North America to size paper. The pica is divided into 12 points. Points are the units that typography is measured in. For example, 10 on 12 is a type specification. Ten points is the size of the characters, and 12 points is the size of the leading or line spacing.
Why should this be so? Well, North American paper sheets are sized in inches because imperial units are what the industry started with. Originally, all the paper-handling machinery built for the North American market, whether for paper making or printing, was made to handle paper to be trimmed to whole-number or whole-number fraction dimensions in inches. To change this would require retooling or replacing most of the paper-handling machinery across this continent, a terrible waste of materials and effort.
Type is measured in points because early lead typesetters had their own system of measurement. They needed a whole-number measuring system with very small units so that they could cast and set type in easy-to-reference sizes. Since lead type is no longer common, keeping this system may seem somewhat arbitrary until you consider that these type sizes have a useful relationship to imperial paper sizes.
Conveniently, point and pica measures correspond to imperial measures, so that 6 picas equal 1 inch. Because of this, picas, points and inches are easy to use together. A half-inch is 3p. A quarter-inch is 18 points or 1-1/2 picas or 1p6. An 8th of an inch is 9 points. (Picas and points are expressed <pica>p<points>, so 4p9 is 4 picas and 9 points or 4-3/4 picas.) These kinds of relationships make design and layout easier, because they help to keep measures simple and easily expressed in whole numbers and whole-number fractions.
Whole numbers are the ones you use most commonly. They are 0, 1, 2, 3 and so on. Anything that simplifies math is welcome, and whole numbers and whole-number fractions like 1/2 are the easiest measures to work with because they simplify math. If you have a document that is 8-1/2 inches wide, the outside margin is 4p, and the inside margin is 5p6 with an 11p narrow column and a 18 pt gutter, it is easy to figure that the remaining text column is 29p6. This sort of grid is relatively easy to set up, and if any changes are made, they will be easy to implement.
The same thing applies with type measures. Ten-on-twelve-point type is easier to work with than 0.1389 inch on 0.1667 inch or 3.528 mm on 4.233 mm type. You can easily see that 14 lines of 12 pt leading are 14 picas deep. It is not so easy to figure 14 times 0.1667. The same thing applies to all type parameters. The font is sized in points, so the job is easier to work with if the leading, indents and spacing, tabs and so on are in the same units.
I once worked on an annual report that was specced as A4, which is a European size, 210 mm by 297 mm. In imperial units, this is a very awkward 8.2677 inches by 11.6929 inches. Since paper sheets here are sized so that signatures can be cut down to imperial sizes, very commonly 8-1/2 by 11 inches, this design decision required that larger paper be purchased and cut down, increasing waste and cost, just to produce a booklet in an unusual size. Then, as the job neared completion, the designer insisted that it was the wrong size. It turned out that she believed that A4 was an imperial size 8-1/4 inches by 11-1/2 inches, not a metric size at all. This required considerable extra expense to change. Unless you are ordering European-sized paper and your printer is set up to print and trim to metric specifications, millimetres have no place in design and are just going introduce the possibility of expensive problems.
The point of all this is that while paper is made and handled in imperial sizes, in design, it should be specced in simple inch measures. Type and typography, including specifications for such things as margins and gutters, as well as type-style specifications, should be handled in points and picas. And in all cases, where possible, measures should be in whole numbers and simple whole-number fractions like 1/2, 3/4 or 7/8 for inches or 36pt, 4p6 or 5p3 for the corresponding pica measures. Don’t be the person who creates documents with arbitrary, awkward dimensions that are difficult to work with and that drive people like me mmm-mmmm-mmm-m mad.