In letterpress every part of the print has to be defined by a physical object, including any white space you see. This requires a vast inventory of physical ‘spacing’ systematically ordered according to size in increments of 1 point or 1/72 of an inch. 12 points make 1 pica. Larger spacing in widths of 2, 3, 4, 5, 6 pica etc. and lengths of 10 - 48 pica is known as furniture and is stored in a compartmentalised furniture rack. Most people only see type but in letterpress furniture is a fundamental tool.
The image below shows a single asterisk held in place by white space in the form of furniture all around it.
