Note: It is with some trepidation that I reprint the two articles below. My fear is that while they are themselves quite clear, they will further conflate two terms in the minds of the hasty. To make things clear:
Mitering is the well-established operation of cutting printers' leads or rule at an angle. Hand-operated mitering machines became standard items in the 19th century and many traditionalist printers still use them today. Mitering could also be done on a printers' saw with appropriate attachments. Finally, the H.B. Rouse Company made a very nice power mitering machine. This machine is still relatively common among letterpress printers.
Mortising is an entirely different operation: the cutting of rectilinear bits off of individual types so as to be able to set them more closely together. Rouse also made a mortising machine. Moreover, since the mechanical requirements of both mitering and mortising machines are similar (though their functions differ), Rouse at some point re-engineered their mortising machine to share many components with their mitering machine.
The only problem is that the two machines look similar and the words "mortising" and "mitering" sound a bit similar. So, invariably, when the subject of mortising comes up in online discussions someone will pipe up and say they have a mortising machine available. Invariably, what they actually have is a mitering machine. This leads to false hopes and great disappointments for those of us who do not yet have a mortising machine but desire one.
All this having been said, the "mortising" described in the second article below (by Lane) is mortising as generally understood. The "mitering" described in the first article below (by Saladé) has nothing to do with mitering leads or rule as is generally done. Instead, it is a unique application of mitering to accomplish the same ends as mortising, done apparently in ignorance of the practice of mortising. To the best of my knowledge, this mitering of type in place of mortising was never generally practiced.
Mitering of Type (1916)
Saladé, Robert F. "Mitering Type Characters for Fine Work." The Printing Art. Vol. 27, No. 5 (July 1916): 454-456.
Mortising vs. Mitering. (1916)
Lane, Loring. "Mortising vs. Mitering Type Characters." The Printing Art. Vol. 28, No. 2 (Oct. 1916): 102-104.
The process of mortising is shown as a part of high-quality composition in a film from the 1940s, Hand Composition at R. R. Donnelley & Sons Co. This film is now online at Doug Wilson's PrintingFilms.com website. The portion showing mitering begins at time 3:53 in this copy of the film.
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