On 10 Jun 2014, at 14:29, Philippe Verdy <verdy_p at wanadoo.fr> wrote: > ℕ ⊃ℤ ⊃ℚ ⊃ℝ ⊃ℂ are without doubt more useful and more common in double-struck styles than in Fraktur styles. Fraktur would normally be for Lie algebras. For sets, some other style or none. And logicians use their own notation.