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LaTeX were traditional German sans-serif blackboard-bold
letters, mainly for: N (natural numbers), Z (integers), Q
(rational numbers), R (reals), C (complex numbers).  These five
symbols are used widely in German secondary-school math
textbooks and in a type of book called "Formelsammlung" (a book
that functions as a collection of formulas; usually secondary
school students will use one for math and physics each). The
most widely used Formelsammlung was (for a long time at least)
"Mathematische Formeln / Erweiterte Ausgabe E" by Sieber/Klett.
The standard array of German textbook publishers (such as
Cornelsen and Klett; I'm afraid I don't remember which math
textbook my school used) always styled the blackboard-bold
letters in a particular, uniform way: for sans-serif letters
this was with the doublestroke having equal strength as the one
that it is doubling, and also without a connection to the
original stroke (for vertical-ish doubled strokes