It was pointed out that the isoscalar dipole strength distribution in nuclei contributes to the Schiff moment. The nuclear Schiff moment is essential in the mechanism that induces parity and time-reversal violations in the atom. In this paper, we explore, theoretically, the properties and systematics of the isoscalar dipole in nuclei with an emphasis on the low-energy strength and the inverse energy-weighted sum which determines the Schiff moment. We also study the influence of the isovector dipole strength distribution on the Schiff moment. The influence of large neutron excess in exotic nuclei is examined. The centroid energies of the isoscalar giant dipole resonance and the overtone of the isovector giant dipole resonance are given for a wide range of nuclei.