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The letters a physics page is written in: the quantity symbols of mechanics, waves, optics and electromagnetism. What holds the group together is that almost every member is an ordinary italic letter carrying a meaning nothing about its shape declares, and that most of them are shared — one italic capital B is magnetic flux density in one chapter and susceptance in the next, one Greek mu is friction, permeability, chemical potential and a millionth. Several members exist mainly to separate a pair that look identical and are not: B against H, E against D, impedance against reactance, angular frequency against frequency, the two incompatible things both called the wavenumber.
A Greek mu with a nought subscript, the magnetic constant.
A Greek sigma, relating temperature to radiated power.
An italic capital R, the molar gas constant.
An italic capital F, the charge on a mole of electrons.
A capital R with an infinity subscript, the hydrogen spectrum scale.
An italic a with a nought subscript, the atomic length scale.
A Greek mu with a capital B subscript, the electron's magnetic unit.
A Greek mu with a capital N subscript, the nuclear magnetic unit.
A Greek omega, radians per second rather than cycles per second.
An italic k or a tilde-topped nu, waves per unit length.
A Greek phi or delta, how far into a cycle a wave is.
A bold or italic p, mass times velocity.
A capital L, rotational momentum about an axis.
A Greek tau, or a capital M in engineering.
An italic capital I or J, resistance to angular acceleration.
A script capital L, kinetic energy minus potential energy.
An italic capital S, the integral of the Lagrangian over time.
A Greek mu, the ratio of friction to normal force.
An italic k, stiffness in newtons per metre.
An italic capital Q, how slowly an oscillator loses energy.
An italic n, how much a medium slows light.
An italic f, the distance from a lens to its focus.
A capital D, optical power as a reciprocal metre.
The letters NA, a lens's light-gathering cone.
A theta with a B subscript, the angle of complete polarisation.
A theta with a c subscript, beyond which light cannot escape.
A bold capital B, measured in tesla.
A bold capital H, measured in amperes per metre.
A bold capital D, the electric field with polarisation folded in.
A script capital E, and not a force at all.
A bold capital J, current per unit cross-section.
A Greek sigma, the reciprocal of resistivity.
A Greek rho, resistance scaled out of the shape.
An italic capital Z, resistance extended to alternating current.
An italic capital X, the imaginary part of impedance.
An italic capital Y, the reciprocal of impedance.
An italic capital B, the imaginary part of admittance.
A bold capital S, the direction and rate of energy flow.
A bold capital A, a field whose curl is the magnetic field.