A type of evidence which came up, not infrequently, in murder mysteries from the golden age of detective fiction was the identification of typed notes to the typewriter they came from. Sometimes this was unimportant and often it was misleading, but the evidence was always strong and, moreover, was the sort of evidence which could link things that would have been hard, otherwise, to link. The advent of printers (which could not be identified in this way) was a real blow to detective stories.

That said, I do not know how uniquely identifiable typewriters actually were. It was stated in books from more than one author that each typewriter’s writing was as unique as a fingerprint, and though on one level it makes sense, it does also seem a bit implausible. On the other hand, manufacturing standards were not as strict and the output not as uniform in the early 1900s as they are today. It is plausible that there was a fair amount of variety in the exact shape of the letters, and with fifty two of them (including capitals) and ten numbers, there was a reasonable scope for individual variation.

That said, manufacturing standards weren’t abysmal, and there were an awful lot of typewriters sold. This may be why authors would sometimes give typewriters some more uniquely identifying characteristics, such as a character consistently out of alignment or another which was chipped. Chips, like scars, are fairly unique.

Be that as it may, the thing was certainly accepted in detective stories from the golden age and served their authors well. It could be very handy indeed to find out that a document could not have existed before a certain date because that’s when the typewriter which wrote it was purchased; it could be even more handy to find out that a threatening note was typed on the machine in a particular office to which—in theory—only a few people had access. It’s much harder to do that, these days.

Having said that, it does just occur to me that printers do occasionally leave unique imperfections in the documents that they print. It’s not common, but sometimes when they are failing they will start leaving streaks of various kinds that look the same on every page. I doubt I’d be likely to use this in any of my stories, though, since it’s far more obvious to the person writing the incriminating note than the minor variations used to identify typewriters. Still, it’s worth keeping in one’s back pocket.