  Entanglement is an useful resource because some global operations cannot be
locally implemented using classical communication. We prove a number of results
about what is and is not locally possible. We focus on orthogonal states, which
can always be globally distinguished. We establish the necessary and sufficient
conditions for a general set of 2x2 quantum states to be locally
distinguishable, and for a general set of 2xn quantum states to be
distinguished given an initial measurement of the qubit. These results reveal a
fundamental asymmetry to nonlocality, which is the origin of ``nonlocality
without entanglement'', and we present a very simple proof of this phenomenon.
