There are different widths of spacing for each size of type, all based around the square of the type size body called the Em. The em is then split into halves, thirds, quarters, fifths and sometimes sixths. Half an em is an en. The names of these come from an ‘M’ being roughly square and and ‘N’ being half the width. In ye olde days, compositors in a busy composing room would often mishear because em and en sound similar, so they named them muttons and nuts.

 

There are also coppers and brasses. These are very thin slivers of metal, and do not relate to the body size of the type you are setting. Copper is ½ point and brass is 1 point, allowing you to make tiny adjustments to spaces between letters. Of course, if you want to use an even thinner space you can cut a slip of paper.

 

Wider spaces are called quads and are again based on the em of the body size of type. These generally come in 2, 3 and 4em widths, and are useful for line spaces and filling the ends of lines of text.

 

The table below is a visual guide for each significant body size of type. I have also included a calculation of the width to one decimal place.