space line

A space line is a strip of metal, typically made from lead alloy, that sits between lines of type in letterpress printing to create vertical separation. Unlike type characters themselves, which are raised to accept ink, space lines sit at the height of the body (the shank of the type) and print nothing; they are purely mechanical spacers. They come in fixed thicknesses, typically measured in points, with common sizes being 6pt, 12pt, and 18pt, allowing compositors to build consistent line spacing by stacking them beneath each line of set type.

The material matters because the metal must be soft enough to handle without cracking but rigid enough to hold position in the form without bending under pressure. Lead alloys, sometimes with small amounts of tin and antimony, provided the ideal balance of workability and durability. Space lines wear with repeated use and handling, and once they become warped or develop burrs from the platen pressure of repeated impressions, they affect the uniformity of the printed page and must be discarded.

While their primary function is to separate lines, space lines also served other purposes in the composing room. They could be combined with furniture and quoins to pack out and lock up a forme, filling gaps and preventing type from shifting during the press run. Experienced compositors understood that space line thickness affected not just the visual rhythm of the page but also the mechanical stability of the locked forme.

The term lead persists in printing vocabulary even today, though the material is no longer literally used in modern offset and digital workflows. The vertical space between lines of type became known as leading, which printers still adjust digitally, inheriting both the concept and the name from the metal strips that once made the adjustment physical and permanent.