Tape formulation affects the retention of the magnetic signal, especially high frequencies, the frequency linearity of the tape, the S/N ratio, print-through, optimum AC bias level (which must be set by a technician aligning the machine to match the tape type used, or more crudely set with a switch to approximate the optimum setting.) Tape formulation varies between different tape types (ferric oxide chromium dioxide [CrO2, etc.) and also in the precise composition of a specific brand and batch of tape. (Studios therefore generally align their machines for one brand and model number of tape and use only that brand and model.) Backing material type and thickness affect the tensile strength and elasticity of the tape, which affect wow-and-flutter and tape stretch; stretched tape will have a pitch error, possibly fluctuating. Backing thickness also affects print-through, the phenomenon of adjacent layers of tape wound on a reel picking up weak copies of the magnetic signal from each other. Print-through on analog tape causes unintended pre- and post-echoes on playback, and is generally not fully reversible once it has occurred. In professional half-track use, post-echo is considered less problematic than pre-echo (as the echo is largely masked by the signal itself) and therefore tapes stored for long periods are kept "tails-out", where the tape must be first wound "backwards" onto the take-up spool before playback.