Wire Gauge

I have been making jewelry for the past few months and noticed that wire gauge numbers seem to be opposite of what I thought they would be. If a wire gauge is 12 and another wire gauge is 20 which would you think is thicker? I would have chosen 20. It is just the opposite. The higher the number the thinner the wire. What is the rationale for that? It is the same thing with blood levels for Coumadin. The lower the number the thicker the blood. I guess some variables are topsy turvy.