Ch 8.1 The notation system invented by P.A.M. Dirac using kets $|\psi\rangle$ (states), bras $\langle\phi|$ (dual vectors), and brackets $\langle\phi|\psi\rangle$ (inner products). Representation-independent and universally used in quantum mechanics from Chapter 8 onward. *See also:* Bra, Ket, Inner