Impure s Uncommon

In Italian, an /s/ followed by another consonant. When used immediately before a masculine noun beginning with an impure s, the definite article changes from il to lo in the singular and from i to gli in the plural, the indefinite article changes from un to uno, the partitive article changes from dei to degli, and some adjectives and pronouns also change form., This also happens before words beginning with gn, pn, ps, x, y, and z and before certain words starting with ch that are derived from French.