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Many letters of the Latin alphabet, both capital and small, are used in mathematics, science, and engineering to denote by convention specific or abstracted constants, variables of a certain type, units, multipliers, or physical entities. Certain letters, when combined with special formatting, take on special meaning.

Below is an alphabetical list of the letters of the alphabet with some of their uses. The field in which the convention applies is mathematics unless otherwise noted.

## Aa[edit]

- A represents:
- the first point of a triangle
- the digit "10" in hexadecimal and other positional numeral systems with a radix of 11 or greater
- the unit ampere for electric current in physics
- the area of a figure
- the mass number of an element in chemistry
- the Helmholtz free energy of a closed thermodynamic system of constant pressure and temperature
- a vector potential, in electromagnetics it can refer to the magnetic vector potential
- an Abelian group in abstract algebra
- the Glaisher–Kinkelin constant

- 𝔸 () represents the algebraic numbers or affine space in algebraic geometry
- a represents:
- the first side of a triangle (opposite point A)
- the scale factor of the expanding universe in cosmology
- the acceleration in mechanics equations
- the
*x*-intercept of a line using the line equation - the unit are for area (100 m²)
- the unit prefix atto (10
^{−18}) - the first term in a sequence or series

## Bb[edit]

- B represents:
- the digit "11" in hexadecimal and other positional numeral systems with a radix of 12 or greater
- the second point of a triangle
- a ball (also denoted by ℬ () or 𝔹 ())
- a basis of a vector space or of a filter (both also denoted by ℬ ())
- in econometrics and time-series statistics it is often used for the backshift or lag operator, the formal parameter of the lag polynomial
- the magnetic field, denoted or

*B*with various subscripts represents several variations of Brun's constant and Betti numbers; it can also be used to mean the Bernoulli numbers- b represents:
- the second side of a triangle (opposite point B)
- The impact parameter in nuclear scattering
- the
*y*-intercept of a line using the line equation - usually with an index, sometimes with an arrow over it, a basis vector

## Cc[edit]

- C represents:
- the third point of a triangle
- the digit "12" in hexadecimal and other positional numeral systems with a radix of 13 or greater
- the unit coulomb of electrical charge
- capacitance in electrical theory
- with indices denotes the number of combinations, a binomial coefficient
- together with a degree symbol (°) represents the Celsius measurement of temperature = °C
- the circumference of a circle or other closed curve
- the complement of a set (lowercase
*c*and the symbol ∁ are also used)

*C*represents:- an arbitrary category

- ℂ () represents the set of complex numbers
- A vertically elongated C with an integer subscript n sometimes denotes the n-th coefficient of a formal power series.
- c represents:
- the unit prefix centi (10
^{−2}) - the molar concentration in chemistry

- the unit prefix centi (10
*c*represents:- the speed of light in a vacuum
- the third side of a triangle (opposite corner C)

- Lower case Fraktur denotes the cardinality of the set of real numbers (the "continuum"), or, equivalently, of the power set of natural numbers

## Dd[edit]

- D represents
- the digit "13" in hexadecimal and other positional numeral systems with a radix of 14 or greater
- diffusion coefficient or diffusivity in dimensions of [length^2 / time]
- the differential operator in Euler's calculus notation

- d represents
- the differential operator
- the unit day of time (86,400 s)
- the difference in an arithmetic sequence
- a metric operator/function
- the diameter of a circle

## Ee[edit]

- E represents:
- the digit "14" in hexadecimal and other positional numeral systems with a radix of 15 or greater
- an exponent in decimal numbers. For example, 1.2E3 is 1.2×10³ or 1200
- the set of edges in a graph or matroid
- the unit prefix exa (10
^{18}) - energy in physics
- electric field denoted or
- electromotive force (denoted and measured in volts), refers to voltage.
- an event (as in P(
**E**), which reads "the probability P of event**E**occurring") - in statistics, the expected value of a random variable

- e represents:
- Euler's number, a transcendental number equal to 2.71828182845... which is used as the base for natural logarithms
- a vector of unit length, especially in the direction of one of the coordinates axes
- the elementary charge in physics
- an electron, usually donated e
^{−}to distinguish against a positron e^{+} - the eccentricity of a conic section
- the identity element in a group

## Ff[edit]

- F represents
- the digit "15" in hexadecimal and other positional numeral systems with a radix of 16 or greater
- the unit farad of electrical capacity
- the Helmholtz free energy of a closed thermodynamic system of constant pressure and temperature
- together with a degree symbol (°) represents the Fahrenheit measurement of temperature = °F

*F*represents- force in mechanics equations
is a hypergeometric series_{p}F_{q}- the probability distribution function in statistics
- a Fibonacci number
- an arbitrary functor

- f represents:
- the unit prefix femto (10
^{−15})

- the unit prefix femto (10
*f*represents:- the generic designation of a function

## Gg[edit]

- G represents
- an arbitrary graph, as in:
*G*(*V*,*E*) - an arbitrary group
- the unit prefix giga (10
^{9}) - Newton's gravitational constant
- the Einstein tensor
- the Gibbs energy
- the centroid of a triangle
- Catalan's constant

- an arbitrary graph, as in:
- g represents:
- the generic designation of a second function
- the acceleration due to gravity on Earth

## Hh[edit]

- H represents:
- a Hilbert space
- the unit henry of magnetic inductance
- the homology and cohomology functor
- the enthalpy
- the (Shannon) entropy of information
- the orthocenter of a triangle
- a partial sum of the harmonic series
- magnetic field, denoted .

- H
_{0}represents Hubble's parameter as measures today (100 h km·s^{−1}·Mpc^{−1 }with h being the associated error) - ℍ () represents the quaternions (after William Rowan Hamilton)
- ℋ () represents the Hamiltonian in Hamiltonian mechanics
- h represents:
- the class number in algebraic number theory
- a small increment in the argument of a function
- the unit hour for time (3600 s)
- the Planck constant (6.626 069(57)× 10
^{−34}J·s) - the unit prefix hecto (10
^{2}) - the altitude of a triangle

## Ii[edit]

- I represents:
- the closed unit interval, which contains all real numbers from 0 to 1, inclusive
- the identity matrix
- the Irradiance
- the moment of inertia
- Intensity in physics
- the incenter of a triangle

*I*represents:- the index of an indexed family

- i represents:
- the imaginary unit, a complex number that is the square root of −1
- a subscript to denote the
*i*th term (that is, a general term or index) in a sequence or list - the index to the elements of a vector, written as a subscript after the vector name
- the index to the rows of a matrix, written as the first subscript after the matrix name
- an index of summation using the sigma notation
- the unit vector in Cartesian coordinates going in the X-direction, usual bold
**i**

## Jj[edit]

- J represents:
- the unit joule of energy
- the current density in electromagnetism denoted .
- the Radiosity in thermal mechanics

*J*represents:- the scheme of a diagram in category theory

- j represents:
- the index to the columns of a matrix, written as the second subscript after the matrix name
- in electrical engineering, the square root of −1, instead of i
- in electrical engineering, the principal cube root of 1:

## Kk[edit]

- K represents:
- k represents
- the unit prefix kilo- (10³)
- the Boltzmann constant, this is often represented as k
_{B}to avoid confusion with - the Wavenumber of the wave equation
- an integer, e.g. a dummy variable in summations, or an index of a matrix.
- an unspecified (real) constant
- the spring constant of Hooke's law
- the spacetime Curvature from the Friedmann equations in cosmology

## Ll[edit]

- L represents:
- Length, used often in quantum mechanics as the size of an infinite square well
- Angular momentum
- the unit of volume the litre
- the radiance
- the space of all integrable real (or complex) functions
- the space of linear maps, as in
*L*(*E*,*F*) or*L*(*E*) = End(*E*) - the likelihood function
- a formal language
- the lag operator in statistics
- a Lucas number

- l represents:
- the unit of volume the litre
- the length of a side of a rectangle or a rectangular prism (e.g. V = lwh; A = lw)
- the last term of a sequence or series (e.g. S
_{n}= n(a+l)/2)

- ℒ () represents:
- the Lagrangian (sometimes just L)
- Exposure (in particle physics)

## Mm[edit]

- M represents:
- a manifold
- a metric space
- a matroid
- the unit prefix mega- (10
^{6}) - the Madelung constant for crystal structures held by ionic bonding

- m represents:

## Nn[edit]

- N represents
- the unit newton of force
- the Neutron number
- the Particle number in thermodynamics
- The number of particles of a thermodynamical system
- the nine-point center of a triangle

- N
_{A}represents the Avogadro constant which is the number of entities in one mole (used mainly in the counting of molecules and atoms) and is 6.022 141 79(30) × 10,^{23}mol^{−1} - ℕ () represents the natural numbers
- n represents
- the number of columns in a matrix
- the "number of" in algebraic equations.
- A neutron, often shown as
^{1}_{0}n - the Number density of particles in a Volume
- the unit prefix nano (10
^{−9}) - the nth term of a sequence or series (e.g. t
_{n}= a+(n-1)d) - the principal quantum number

## Oo[edit]

- O represents
- the order of asymptotic behavior of a function (upper bound); see Big O notation
- — the origin of the coordinate system in Cartesian coordinates
- the circumcenter of a triangle or other cyclic polygon, or more generally the center of a circle

- o represents
- the order of asymptotic behavior of a function (strict upper bound); see Little o notation
- the order of an element in a group

## Pp[edit]

- P represents:
- the pressure in physics equations
- the unit prefix peta (10
^{15}) - probability in statistics and statistical mechanics
- an arbitrary point in geometry

- ℙ () represents
- the prime numbers
- projective space
- a probability (as in
**P**(E), which reads "the probability**P**of event E happening")

- p represents

## Qq[edit]

- Q represents:
- heat energy

- ℚ () represents the rational numbers
- q represents:
- the deceleration parameter in cosmology
- electric charge of a particle

## Rr[edit]

- R represents:
- the Ricci tensor
- the circumradius of a cyclic polygon such as a triangle
- an arbitrary relation

- ℝ () represents the set of real numbers and various algebraic structures built upon the set of real numbers, such as
- r represents:
- the radius of a circle or sphere
- the inradius of a triangle or other tangential polygon
- the ratio of a geometric series (e.g. ar
^{n-1}) - the separation of two objects, for example in Coulomb's law

## Ss[edit]

- S represents
- a sum
- the unit siemens of electric conductance
- the unit sphere (with superscript denoting dimension)
- the scattering matrix
- entropy

- s represents:
- an arclength
- the displacement in mechanics equations
- the unit second of time
- a complex variable
*s*= σ +*i**t*in analytic number theory - the semiperimeter of a triangle or other polygon

- 𝒮 () represents a system's action in physics

## Tt[edit]

- T represents:
- the top element of a lattice
- a tree (a special kind of graph)
- temperature in physics equations
- the unit tesla of magnetic flux density
- the unit prefix tera (10
^{12}) - the stress–energy tensor
- tension in physics
- an arbitrary monad

- t represents:
- time in graphs, functions or equations
- a term in a sequence or series (e.g. t
_{n}= t_{n−1}+5) - the imaginary part of the complex variable
*s*= σ +*i**t*in analytic number theory - the sample statistic resulting from a Student's t-test

## Uu[edit]

- U represents:
- a U-set which is a set of uniqueness
- a unitary operator
- in thermodynamics, the internal energy of a system
- a forgetful functor

- U(n) represents the unitary group of degree n
- ∪ represents the union operator

## Vv[edit]

- V represents:
- v represents the velocity in mechanics equations

## Ww[edit]

- W represents:
- the unit watt of power
- work, both mechanical & thermodynamical
- in thermodynamics, the number of possible quantum states in Boltzmann's entropy formula

*w*represents:- the coordinate on the fourth axis in four-dimensional space.

## Xx[edit]

*X*represents*x*represents- a realized value of a random variable
- an unknown variable, most often (but not always) from the set of real numbers, while a complex unknown would rather be called
*z*, and an integer by a letter like*m*from the middle of the alphabet. - the coordinate on the first or horizontal axis in a Cartesian coordinate system, or the viewport in a graph or window in computer graphics.

## Yy[edit]

- Y represents:
- the unit prefix yotta- (10
^{24})

- the unit prefix yotta- (10
*Y*represents:- a second random variable

- y represents:
- the unit prefix yocto- (10
^{−24})

- the unit prefix yocto- (10
*y*represents:- a realized value of a second random variable
- a second unknown variable
- the coordinate on the second or vertical axis (backward axis in three dimensions) in a linear coordinate system, or in the viewport of a graph or window in computer graphics.

## Zz[edit]

- Z represents:
- the unit prefix zetta (10
^{21}) - the atomic number in chemistry and physics
- a standardized normal random variable in probability theory and statistics
- The partition function in statistical mechanics
- in meteorology, the radar reflectivity factor

- the unit prefix zetta (10
- ℤ () represents the integers
- z represents:
- the unit prefix zepto (10
^{−21}) - the coordinate on the third or vertical axis in three dimensional space
- the view depth in computer graphics, see also "z-buffering"
- the argument of a complex function, or any other variable used to represent a complex value
- in astronomy, wavelength redshift
- a third unknown variable

- the unit prefix zepto (10