in
The 96-pixel inch — the link that ties the whole system together.
One in is worth exactly 2.54 centimeters and exactly 96 CSS pixels — and it is from that second equality that everything else derives. The inch is the vault's primary standard: cm, mm, Q, pt and pc are all defined as fractions of it.
The right choice is the same as its metric sisters': print, where 6in by 4in photos and half-inch margins mean what they say. On screen, it is mostly the system's walking conversion table.
.foto {
width: 6in;
height: 4in;
}