The doublestruck capital letter Q, 
, denotes the field of rationals.
It derives from the German word Quotient, which can be translated as "ratio."
The symbol 
first appeared in Bourbaki's Algèbre
(reprinted as Bourbaki 1998, p. 671).
Q
See also
C, C-*, Field of Rationals, I, N, Q-+, Q-Bar, Q-Function, q-Product, q-Series, R, ZExplore with Wolfram|Alpha
References
Bourbaki, N. Éléments de mathématique: Algèbre. Reprinted as Elements of Mathematics: Algebra I, Chapters 1-3. Berlin:Springer-Verlag, 1998.Dummit, D. S. and Foote, R. M. Abstract Algebra, 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, p. 1, 1998.Referenced on Wolfram|Alpha
QCite this as:
Weisstein, Eric W. "Q." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Q.html