At this point one might have noticed that both the wave function ^(r, t) and the expansion coefficients have been referred to as probability amplitudes. In this chapter, we introduce the Dirac notation, a way of writing vectors which reveals that y/(r, t) and C*(/) are both expansion coefficients of the state vector | y/{t) >, a vector in an abstract space. The absolute squares of the expansion coefficients are the probabilities of finding the state vector in corresponding eigenstates. The Dirac notation also provides a compact, versa­ tile means for considering any (and all) representatives (energy, position, etc.) o f a state vector.