The "double contour" style the capitals N, Z, Q, R, and C are written in when they are meant to mean the set of natural numbers, integers, reals, rationals, and complex numbers.

The background of this unsual style is interesting. Initially, these letters were printed in bold type. However, it is difficult to illustrate bold in handwriting, so they were written "double". This convention was then exported back into printed books, and is now a nuisance for anyone who has to typeset mathematics.