Leading is a term that has made it’s way to the graphic designer’s computer. It defines the gap between lines and in letterpress printing it is done by inserting a thin strip of lead.
Leads are around the same height as spacing and so don’t print. They vary in length and also width. On width, leads are supplied in point sizes and are most commonly available in 1, 1½, 2 and 3 points. It can be confusing to determine the difference between different sizes when composing, so in some cases it might be wise to standardise on one or two sizes. It’s difficult to detect the 0.007″ between 1 and 1½pt leads. On length, they should be accurately cut and your supplier will normally do this. If you are cutting then you need to be as precise as possible. It is also worthwhile to cut leads ½pt shorter than the page measure used to help overcome the problems of squeeze.
Be careful in your use of leads: Perpetua is the best example of a small x‑height face and Plantin has a large x‑height meaning they look smaller and larger than each even when on the same body (of, say, 12pt). Adding leading to Perpetua will lighten the page; and having no leading in Plantin will make for a very dark page. There are no standards to say what leading will work for each page, that is a matter for you as the typographer.
Leads are normally stored in racks with different compartments for each length and sizes so that they can be easily found. Remember that leads can also be combined: there is no need to keep a 40em lead when two 20em leads can sit next to each other. If you are short of space you could standardise here on 6em multiples so that you can cover 6, 12, 18, 24 and 30 ems which should account for most practical uses.
Clumps can be thought of as extra wide spaces: they are usually made in multiples of an em and so 12pt clumps will normally be 12, 24, 36 or 48pt wide. They are made of the same material as spaces and are the same height. A secondary use here is in word spacing for larger type sizes: a 12 x 36 pt clump makes a useful space for 36pt type.
Reglet is line spacing that is larger than leads. Almost always made of wood and the same height as leads, it is used to give more space between lines and is most often employed on title pages or notices to create white space between lines. Again, this is often supplied pre-cut but can be supplied in long lengths to be cut by the printer. Adana sold a small saw and mitre block for this purpose.
Next in terms of size are quotations, originally designed to correctly space side-notes and quotations in book work. They are metal and full spaces that are multiples of clumps. Typically 48pt and larger they are an accurate way of filling space.
Furniture is the largest of the spacing material. Originally wood, it is supplied in ems rather than points. Finding wood to suffer from warping and easily damaged, printers began to use metal furniture (from the same material as spaces), and then aluminium girder pattern furniture which was lighter. One development was the use of a Formica-derived material called Resalite. Stephenson, Blake claimed this was better than metal furniture because damage on one dimension would not cause problems with accuracy.
Again, you might have the opportunity to standardise on one or two widths and 6em multiples of length to save space and time.
This rather grand sounding material is used to set curved lines of type. It’s not used too often now, but the was a fad for firm names to be set in an arc or circle. These are always supplied in pairs and have two surfaces that match each other. Type is set between them and then locked up. There are a number of methods used to get this to work correctly: setting type against sticky tape to hold them in place; or spacing them with wet paper to form a sold mass once dry.