Global phase

A global phase is an overall factor in front of every amplitude, such as a minus sign applied to the whole state. No measurement can see it, which is why the puzzle on this site treats two columns that differ only by that factor as the same answer.

Keith lost an afternoon to an overall minus and called the bench unfair. The bench was following the rule, and the rule is now written on the inside cover of the lab book, where the argument used to start.