Preferred basis. In quantum mechanics, the wave function can be expanded on arbitrarily different bases. Still, we finally observe only a few bases, such as position. One of the biggest successes of the decoherence research program has been establishing that the continuous monitoring of the system by an environment selects a preferred system basis, the pointer states. These are the states with the least entanglement with the environment and that, therefore, survive to the decoherence.