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In maths, double-struck letters are used every day. Today I’ve found out they have a quite interesting history. From the Wikipedia article:

[Blackboard bold] originated from the attempt to write bold letters on blackboards in a way that clearly differentiated them from non-bold letters, and then made its way back in print form as a separate style from ordinary bold, possibly starting with the original 1965 edition of Gunning and Rossi's textbook on complex analysis. Some mathematicians, therefore, do not recognize blackboard bold as a separate style from bold: Jean-Pierre Serre, for example, has publicly inveighed against the use of "blackboard bold" anywhere other than on a blackboard, and uses double-struck letters when writing bold on the blackboard.

And from another discussion:

[Such] symbols were first used in polycopied/mimeographed notes, and similar materials prepared on typewriters. Although (as I recently mentioned in another thread in c.t.t) there was at least one office typewriter (an Olympia) for sale by the mid 1960s with a built-in "poor man's bold" doublestrike function, for a long time--until the era of Typ-It insertible type bars, and not too much later the IBM Executive and Selectric lines of typewriters--about the best that could easily be done to fake "bold" was to overstrike an uppercase I (or a single quote) on your R or C. Given a Selectric, one had the further option of doublestriking your R, C, Z, Q, or N with a slight offset (this involved manual intervention with the device that carried the "golf-ball" type element, and was very hard to do consistently).

In a world of mechanical typesetting, a certain amount of improvisation was necessary to overcome the limitations of the physical media. I guess that Typ-It insertible type bars were a little before my time: I began operating dedicated strike-on machines in 1966, when the Varityper 1010F had been well-established for years, and the IBM Selectric Composer was just emerging. Fake bolding wasn’t necessary on either of these machines if you purchased a boldface type element.