The final decisions of the United States Tax Court are res judicata and conclusive as to all matters decided therein as well as all matters which could have been decided. Gibbs v. Commissioner, 673 F. Supp. 1088, 1092 (N.D.Ala. 1987); United States v. Cohn, 682 F. Supp. 209, 223 (S.D.N.Y. 1988). When Mr. Egbert went before the tax court the actual issue was his entire tax liability for the taxable period 1978, 1979 and 1980. The tax court proceeding encompassed all phases of Egbert's income tax liability for those calendar years.