Thus direct and indirect taxes are not always distinguishable; but in the tax systems of the United States most state and municipal taxes are assumed to be direct, because levied upon persons and more permanent forms of property, while the taxes of the general government are by the Constitution indirect, unless levied upon the states according to population. They are in the form of customs or excise, in which some article of commerce or some service rendered gives a value upon which the tax may be transferred. Thus the state, the county, the city and the school district levy upon assessment of property and enumeration of polls. The United States collects upon imported goods of various kinds, upon special articles of manufacture, upon persons or corporations carrying on particular business, and upon commercial transactions of various kinds. _Assessment of direct taxes._—Assessment implies an enumeration of property in the possession of supposed owners and an appraisement of its value. The officer making the assessment is under constraint of an official oath to give a fair valuation. The market price is supposed to control his judgment, and is usually explicitly named in law. In actual practice in various states of the Union assessed valuation often falls as low as one-third or even one-fifth of a fair estimate at market value. This is brought about by several causes. Each assessor fears over-valuation, lest his district will bear too large a share of more general expenses; and his successor is inclined rather to lower than raise the standard of value, from neighborly interest. Even if the assessors of an entire county agree upon terms of valuation, they are together under the same influence with reference to state and special taxes. A more definite cause of under-valuation is the practice of exempting a certain limited amount of property from all taxes. If personal property worth $200 is exempt from taxation for every householder, the smaller the assessment for his total property, the larger in proportion is the exemption. Specific taxes at a fixed rate, for state or school or improvement purposes, operate in the same way to force down the valuation of property in the entire state or district. In assessment of real estate even greater violence is sometimes done to equity. In newly settled portions of the country the valuation of land held in the name of non-resident owners is notoriously high. Often in cities the assessor is subject to political influences and social connections in such a way as to destroy all equity in taxation. The official oath attached to such assessments is a sham.