What must always be true if a covalent bond is to be polar?

If a covalent bond is polar the electronegativity difference between the elements of the compound is between 0.4 and 1.7

Explanation:

The electronegativity measures the power of an atom to attract electrons; if the electronegativity difference between the elements of the compound is big (more than 1.7) the compound will be ionic, that means, an atom or a group of atoms will give electrons and another atom of group of atoms will receive them; if the electronegativity difference is small (less than 0.4), the electrons will be shared by the atoms in a equitative way, i. e. the electrons will spend similar amount of time in the orbitals of each atom; but if the electronegativity difference is between 0.4 and 1.7  a covalent polar bond will be formed, that means valence electrons will spend more time in the orbitals of the most electronegative element.