How to quickly insert Double strike or Blackboard bold symbols in Word equations
To type the symbols in Double strike or Blackboard bold in the equation Microsoft Word (to insert equation into your text, click Alt+=), do one of the following:
 Under Equation Tools, on the Design tab, in the Symbols group,
click the More button:
In the top of list of symbols choose Scripts:
In the Scripts list, in the DoubleStruck group, choose the symbol you prefer:
 Type \double + capital or lowercase letter:
Capital letters in the Double strike or Blackboard bold:
Symbol  Code  Description 

\doubleA  Represents the algebraic numbers or affine space in algebraic geometry. Occasionally represents the algebraic numbers, the algebraic closure of ℚ (more commonly written or ), or the algebraic integers, an important subring of the algebraic numbers. 

\doubleB  Sometimes represents a ball, a Boolean domain, or the Brauer group of a field.  
\doubleC  Represents the set of complex numbers.  
\doubleD  Represents the unit (open) disk in the complex plane (and by generalisation 𝔻^{n} may mean the ndimensional ball) — for example as a model of the Hyperbolic plane. Occasionally 𝔻 may mean the decimal fractions (see number) or splitcomplex numbers.  
\doubleE  Represents the expected value of a random variable, or Euclidean space, or a field in a tower of fields, or the Eudoxus reals.  
\doubleF  Represents a field. Often used for finite fields, with a subscript to indicate the order. Also represents a Hirzebruch surface or a free group, with a subset to indicate the number of generators (or generating set, if infinite).  
\doubleG  Represents a Grassmannian or a group, especially an algebraic group.  
\doubleH  Represents the quaternions (the H stands for Hamilton), or the upper halfplane, or hyperbolic space, or hyperhomology of a complex.  
\doubleI  The closed unit interval or the ideal of polynomials vanishing on a subset. Occasionally the identity mapping on an algebraic structure, or an indicator function, or the set of imaginary numbers (i.e., the set of all real multiples of the imaginary unit, more commonly indicated i ℝ).  
\doubleJ  Occasionally represents the set of irrational numbers, R\Q (ℝ\ℚ).  
\doubleK  Represents a field, typically a scalar field. This is derived from the German word Körper, which is German for field (literally, "body"; cf. the French term corps). May also be used to denote a compact space.  
\doubleL  Represents the Lefschetz motive.  
\doubleM  Sometimes represents the monster group. The set of all mbyn matrices is sometimes denoted 𝕄(m, n).  
\doubleN  Blackboard bold capital N (for natural numbers set).  
\doubleO  Represents the octonions.  
\doubleP  Represents projective space, the probability of an event, the prime numbers, a power set, the irrational numbers, or a forcing poset.  
\doubleQ  Blackboard bold capital Q (for rational numbers set).  
\doubleR  Represents the set of real numbers. ℝ_{>0} represents the positive reals, while ℝ_{≥0} represents the nonnegative real numbers.  
\doubleS  Represents a sphere, or the sphere spectrum, or occasionally the sedenions.  
\doubleT  Represents the circle group, particularly the unit circle in the complex plane (and 𝕋^{n} the ndimensional torus), or a Hecke algebra (Hecke denoted his operators as T_{n} or 𝕋_{ℕ}), or the tropical semiring, or twistor space.  
\doubleU  
\doubleV  Represents a vector space or an affine variety generated by a set of polynomials.  
\doubleW  Occasionally represents the set of whole numbers (here in the sense of nonnegative integers), which also are represented by ℕ_{0}.  
\doubleX  Occasionally used to denote an arbitrary metric space.  
\doubleY  
\doubleZ  Represents the set of integers. 
Some lowercase letters in the Double strike or Blackboard bold:
Symbol  Code  Description 

\doublea  
\doubleb  
\doublec  
\doubled  May represent the differential symbol.  
\doublee  Occasionally used for the mathematical constant e.  
\doublef  
\doubleg  
\doubleh  
\doublei  Occasionally used for the imaginary unit.  
\doublej 
See also this tip in French: Comment insérer rapidement des symboles double barre ou blackboard gras dans l'équation Word.