A wave packet analysis was a key in proving the existence of the scars, and it is still a valuable tool to understand them. In the original work of Heller, the quantum spectrum is extracted by propagating a Gaussian wave packet along a periodic orbit. Nowadays, this seminal idea is known as the linear theory of scarring. Scars stand out to the eye in some eigenstates of classically chaotic systems, but are quantified by projection of the eigenstates onto certain test states, often Gaussians, having both average position and average momentum along the periodic orbit. These test states give a provably structured spectrum that reveals the necessity of scars. However, there is no universal measure on scarring; the exact relationship of the stability exponent χ χ to the scarring strength is a matter of definition. Nonetheless, there is a rule of thumb: quantum scarring is significant when χ χ < 2 π π , and the strength scales as χ χ − − 1 . Thus, strong quantum scars are, in general, associated with periodic orbits that are moderately unstable and relatively short. The theory predicts the scar enhancement along a classical periodic orbit, but it cannot precisely pinpoint which particular states are scarred and how much. Rather, it can be only stated that some states are scarred within certain energy zones, and by at least by a certain degree.