substance-over-form rule

Neither state courts nor Legislatures, by giving the tax a particular name, or by using some form of words, can take away our duty to consider its nature and effect. If by varying the form—that is to say, if, by using one name for a tax instead of another, or imposing a tax in terms upon one subject when another is in reality aimed at—the substance and effect of the imposition may be changed, constitutional limitations upon powers of taxation would come to naught.