{blackboard} is about the particular "double-stroke" font used, for example, to denote the sets of natural, rational, and real numbers in mathematics
blackboard is about the particular "double-stroke" font used, for example, to denote the sets of natural, rational, and real numbers in mathematics (not to be confused with "calligraphic" or "fraktur" fonts).
There are several ways to get blackboard bold letters in LaTeX; see this answer for a table.
- If you use amsmath (or load amsfonts or amssymb), uppercase blackboard bold characters are available through
\mathbb{ABC}. - The
txfontsandpxfontspackages define two different styles of uppercase blackboard bold characters,\mathbband\varmathbb. - The
bbold,mathbbol, andmbboardpackages define sans-serif blackboard uppercase letters, lowercase letters, and digits with\mathbb{ABCabc123}. - The
bbmpackage defines serif blackboard bold letters, numbers, and digits with\mathbbm{ABCabc123}, sans-serif with\mathbbmss{ABCabc123}, and monospaced with\mathbbmtt{ABCabc123}.
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