Lectures. Bra-ket notation. In quantum mechanics, bra–ket notation is a standard notation for describing quantum states, composed of angle brackets and vertical bars.
It can also be used to denote abstract vectors and linear functionals in mathematics. It is so called because the inner product (or dot product on a complex vector space) of two states is denoted by a ⟨bra|ket⟩, Www.users.csbsju.edu/~frioux/dirac/dirac.pdf.