impure s

Wiktionary

1. n. 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. gn, pn, ps, x, y, z, and certain words starting with ch that are derived from French, are treated similarly.