This property numerically defines the pay period to be used in calculations. This value defaults to 12, which corresponds to a monthly pay period. The Earnings property and the TaxAmount method are expressed in terms of this same pay period.

For example, suppose we have

   CTaxControl tc = new CTaxControl();
   tc.DataFilename = "us.dat";
   tc.SelectedTax = "fit";
   tc.PayPeriodsPerYear = 12;   
   tc.Earnings = 1000.00;

giving a TaxAmount of $150. The Earnings and TaxAmount are both monthly figures.

If we were to change PayPeriodsPerYear to 24:

   CTaxControl tc = new CTaxControl();
   tc.DataFilename = "us.dat";
   tc.SelectedTax = "fit";
   tc.PayPeriodsPerYear = 24;   // now 24 for semi-monthly
   tc.Earnings = 1000.00;

the $1000 Earnings value would now correspond to a semi-monthly figure, and TaxAmount would produce a new semi-monthly tax figure, in all likelihood not $150. (This is because most taxes are based on brackets which determine the tax rate.)