  Spin-polarization is known to lead to important {\it magnetic} and {\it
optical} effects in open-shell atoms and elemental solids, but has rarely been
implicated in controlling {\it structural} selectivity in compounds and alloys.
Here we show that spin-polarized electronic structure calculations are crucial
for predicting the correct $T=0$ crystal structures for Pd$_3X$ and Pt$_3X$
compounds. Spin-polarization leads to (i) stabilization of the $L1_2$ structure
over the $DO_{22}$ structure in Pt$_3$Cr, Pd$_3$Cr, and Pd$_3$Mn, (ii) to the
stabilization of the $DO_{22}$ structure over the $L1_2$ structure in Pd$_3$Co
and to (iii) ordering (rather than phase-separation) in Pt$_3$Co and Pd$_3$Cr.
The results are analyzed in terms of first-principles local spin density
calculations.
