Herman Snellen published his chart in Utrecht in 1862 and, without meaning to, wrote one of the strictest type specifications in history. Each optotype sits on a 5×5 grid where stroke width equals the white gap and both equal one fifth of the letter height. At six metres the 20/20 line stands 8.73 mm tall and every stroke subtends exactly one arcminute.

Nothing on the chart is decoration. The white between the rows is measured space, the surface holds a luminance spec, and the letters come from a closed set of nine. Design in this system means treating layout as instrumentation: black ink, white card, and numbers you could check with a ruler.