What are slugs and bleeds and how are they used?

15 Feb 2015

Whilst slugs and bleeds might sound like things used by medieval doctors to cure all manner of nasty ailments, these are actually two terms used in the creation of documents for printing. Previously only really known by those in the trade, with the increase in the use of desktop publishing and design software, as well as significant advances in home printing, these two terms have become more commonly used. So what do they mean?

Bleed Basics

Page bleed refers to printing that extends beyond the edge of a document before it is cut to the final size. When designing a document, it is useful to have this extra space to allow for any inconsistencies in the design, the printing or the cutting. For example, when printing a colourful brochure, it would not be desirable to have any white areas showing where the design finished just short of the edge. Since achieving a perfectly coloured edge all round would require extremely high levels of precision, it is instead much easier to print a little over the edges of what is required. The final edge where the paper will be cut is called the trim line – with this extra printing just over it referred to as the bleed. Typical bleed sizes vary between 2mm and 5mm and are used to ensure each document or brochure has the correct finish.

The Origins of the Slug

Slugs were first used during the early days of typesetting. Made either of lead, a brass alloy or type metal made from a combination of lead, antimony or tin, they were essentially a spacing tool. Ottmar Mergenthaler was a German-born inventor who in the 1880s in America created the pioneering Linotype. This machine was the first mechanised device for typesetting – a typecasting compositor that cast thin lines, or slugs. Matrices of characters and signs were selected via a keyboard – a very early kind of industrial typewriter. Thus, in typography terms, a slug denotes the space taken up by one single line of type.

Modern Slugs

As printing has changed over the centuries, so has the meaning of the term slug. At its most basic, the slug is a section on the document that contains information. This can be as simple as the name of the file, its author and the date of creation or more detailed technical instructions. A slug is useful in a document that is shared or edited by several different people, as it can keep track of all the various revisions that have been made. An online document or web page may also include a slug, although this will usually be removed before the page goes live to the public. When designing documents for printing, the slug will be outside the trim line in the bleed area, usually but not necessarily in the top left or bottom right corners. This is so the information can be seen whilst the document is being edited but will not be visible when the paper is cut to size after printing.

Whilst bleeds and slugs may be unfamiliar, at least as printing terms, they serve a vital function in the preparation of documents, and their implementation is essential in order to create a professional-looking finish.