Well before the days of the Internet, a way was needed so that any local administration could create its own set of letters, in any required size, in a standard, legible manner, in isolation from such niceties as a set of standard stencils bought from somewhere or other - no doubt, with someone making a pretty pfennig out of it.
So, in steps the idea of a typeface that can be made up using a ruler and a pair of compasses, based upon a standard grid that anybody could make, if they knew the simple formula for each character. With only straight lines and sections of circles to use, it would be relatively easy to make up any character at any size.
This is the DIN 1451 font. The characters in it are multiples (ie, whole numbers and then fractions like 1/2 and 1/4, and that's it) of a base unit. The curves are all the same set of integer diameters so again, 1, 2, 3 and so on.
One result of this is that the normal extension of rounded forms (such as the letter 'o' or the round part of the letter 'd') beyond there neighbouring straight lines (such as the flat end of the vertical line in the letter 'd') is not there. However, in today's world of 'one size fits all' it has been forgotten that the larger the type, the smaller this effect is. So whilst a font at 8pt will have a quite larger 'o', by the time you get up to a road sign with six inch letters, the effect is minimal. This is why printing large type using a font that is meant for small type is wrong - that one-size-fits-all Helvetica that looks okay on a business card is no good on a large sign or in small print.
So, where do we start from with DIN 1451?