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The original was posted on /r/todayilearned by /u/Square-Singer on 2025-09-15 08:20:37+00:00.

Original Title: TIL that “Blackboard Bold” (the style of writing used to represent number sets in maths, e.g. ℕ, ℚ, ℝ, or ℤ) only first emerged in the 1950s due to people “double striking” letters on a typewriter to make them bold. It subsequently got into maths in the 70s and onward.