The tax for any calendar year is measured by the amount of wages paid by the employer during such year with respect to employment after December 31, 1938. (See § 31.3306(b)-1 , relating to wages, and §§ 31.3306(c)-1 to 31.3306(c)-3 , inclusive, relating to employment.)
26 C.F.R. §31.3301-2