When you use a term every day, you accept it without question: it’s what we always say, isn’t it? But when you come to think about it, trim size is a slightly odd term to use to describe the dimensions of a book. What it literally means is the size of the book after it has been trimmed during the binding process. Probably we should really talk about trimmed size, but I guess that’s hard to enunciate.

The etymology of trim is obscure. The Oxford English Dictionary tells us “Old English had a verb trymman or trymian < *trumjan to make firm or strong, strengthen, confirm, set (a force) in array, settle, arrange, etc.” though this word went underground (as far as printed sources are concerned) during the Middle English period, reappearing as if a completely new word in the 16th century. They speculate that it may have survived in oral communication.

Trim in this sense of “get ready/ arrange properly” can be seen in the nautical term indicating a sort of shipshape and Bristol condition of a boat or of its sails, or the rearrangement of cargo etc. to achieve such balance. This meaning has been figuratively transferred to the Vicar of Bray sort of trimmer, and to the thin or fit figure. It’s maybe not a big leap from fixing up a ship to adding rickrack to a garment — “Miss Kitty trimmed up her best cap”, and of course from there to what may be its most common usage nowadays: asking the butcher (if you’re lucky enough still to have a butcher) to cut off the excess fat, or getting the barber to restrain his eagerness and just give you a not-too-short back and sides. Thus like so many words when you get down to it, trim refers to both sides of an antithesis: to add stuff and to subtract stuff.

A book needs to be trimmed because when you fold a sheet of paper some of the sides will be closed by their folds. You need the ones down the spine edge in order to sew the book together, but the ones on the outside just get in the way. Olden days would see binders (hand binders) often leaving the folds in situ, and readers sitting with a paper knife as they worked their way through a volume, but helpful publishers/printers quickly started removing this obstacle as soon as they became the ones responsible for putting the book into its case. Later on a sort of antiquing impulse lead some publishers to sell their books bound but with uncut folds. Trimming is done by a guillotine (nowadays usually built into the binding line) which chops off ⅛” on three sides. Thus the trimmed size is ⅛” narrower and ¼” shorter than the pages that came off the press. Why we book people feel the need to make this distinction when we refer to a book’s size is one of those mysteries nobody can fathom. My prime suspect is an addiction to any kind of technical jargon which may necessitate our having to explain to the innocent just how impressive it is that we manage to do our really complicated jobs.

Now those readers who have worked their way behind the jargon here will perhaps have figured out that many books today don’t really need to be trimmed. If the book is printed print-on-demand on a press like a Xerox DochTech which delivers single leaves, there’s no essential need to trim anything. Just because we always do it though, we continue to do it, justifying the minuscule expense by saying pretty spurious things like “Well the pages might have been bashed on their edges on the way from printing to binding” or “Any unevenness of the book edges will be a blemish”, and “If we don’t do it that way the cover might be wider/narrower, taller/shorter than the pages”. In the end I think we do it just because we do it: which course is why so much of what we do still gets done.

See also Standard trim sizes.