  Nonoscillatory second order differential equations always admit ``special'',
principal solutions. For a certain type of oscillatory equation principal pairs
of solutions were introduced by \'A. Elbert, F. Neuman and J. Vosmansk\'y, {\em
Diff. Int. Equations} {\bf 5} (1992), 945--960. In this paper, the notion of
principal pair is extended to a wider class of oscillatory equations. Also an
interesting property of some of the principal pairs is presented that makes the
notion of these ``special'' pairs more understandable.
