  A comparison is made between the off-diagonal and helicity spin bases for top
quark pair production at the FNAL Tevatron. In the off-diagonal basis, 92% of
the top quark pairs are in the spin configuration up-down plus down-up, whereas
in the helicity basis only 70% are left-right plus right-left. The off-diagonal
basis maximizes the spin asymmetry and hence the measured angular correlations
between the decay products, which are more than twice as big in this basis as
compared to the helicity basis. In addition, for the process q qbar --> t tbar,
we give a very simple analytic expression for the matrix element squared which
includes all spin correlations between the production and subsequent decay of
the top quarks.
