  We write down the four-dimensional fully differential decay distribution for
the top quark decay $t \to Wb \to \ell \nu b$. We discuss how its eight
physical parameters can be measured, either with a global fit or with the use
of selected one-dimensional distributions and asymmetries. We give expressions
for the top decay amplitudes for a general $tbW$ interaction, and show how the
untangled measurement of the two components of the fraction of longitudinal $W$
bosons - those with $b$ quark helicities of $1/2$ and $-1/2$, respectively -
could improve the precision of a global fit to the $tbW$ vertex.
