\newcommand{\R}{{\textsf{R}\hspace*{-0.9ex}%
\rule{0.15ex}{1.5ex}\hspace*{0.9ex}}}
\newcommand{\N}{{\textsf{N}\hspace*{-1.0ex}%
\rule{0.15ex}{1.3ex}\hspace*{1.0ex}}}
\newcommand{\Q}{{\textsf{Q}\hspace*{-1.1ex}%
\rule{0.15ex}{1.5ex}\hspace*{1.1ex}}}
\newcommand{\C}{{\textsf{C}\hspace*{-0.9ex}%
\rule{0.15ex}{1.3ex}\hspace*{0.9ex}}}
are almost acceptable at normal size if the surrounding text is
cmr10 (the position of the vertical bar can be affected by
the surrounding font). However, they are not part of a proper maths font,
and do not work in sub- and superscripts. As we’ve seen, there are
plenty of alternatives: that mythical “lazy” person can inevitably
do better than the macros, or anything similar using capital ‘I’
(which looks even worse!). Voluntary (La)TeX effort has redefined
the meaning of laziness (in this respect!).
This answer last edited: 2013-12-04
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=numbersets