The rate of the alcabala varied over time, from as low as two percent to as high as fourteen percent. It was by no means equally imposed everywhere: rates would differ in various parts of the empire, certain goods would be exempted from the alcabala (sometimes because they were considered subsistence goods, sometimes because they fell under a different tax), and tax farming often led the royal treasury to contract with a city government or merchant guild to collect the tax in a particular geographical area and pay a fixed sum to the Spanish treasury. There were numerous specific exemptions such as (from the time of Philip II onwards) horses and mules, hunting birds, and books.