Ems and ens, muttons and nuts

Printers do not measure their types in inches or centimetres but in "points".

There are roughly 72 points to an inch (more precisely 0·9962").

Twelve points, still referred to by the old name of "pica", is a unit used for measuring the lengths of lines of type.

There is also the "em". This is the square of any body size -, it is correct to speak of 6 pt. ems, of a 10 pt. em, and so on - but if no size is referred to then the "em" meant is the 12 pt. ot Pica em.

Half an "em" is known as an "en"; these words sounding so alike many printers refer to them as "mutton" and "nut" respectively.

Below the em and en there are other standard spaces; as fractions of the "em" they are: 1/3 em or "thick", 1/4 em or "middle" and 1/5 em or "thin"; there is also the "hair" space of 1/12 em.

From http://www.dozenalsociety.org.uk/history/points.html