What is a treaty of double taxation?

In a way, this sort of a treaty would make more sense if
it were called a treaty against double taxation.  This is
much more like what the treaty actually does.  What such a treaty does is to make sure
that someone does not get taxed twice on income earned in a
foreign country.

Imagine that you are an American and you
do some work in Japan.  Let's say that the Japanese government takes taxes out of the
pay you get for your work while in Japan.  It would be unfair for you to then pay taxes
in the US on that same income, right?

A treaty of double
taxation prevents that.  It says that the worker will only get taxed in their own
country, not in the country where they are working for just a short
while.