16th century = sedicesimo secolo Il Cinquecento ( a shorter way to say Millecinquecento) because is from 1500 till 1600. It's the same with other centuries from 1200 till 2000 20th century = ventesimo secolo (from 1900 till 2000; Il Novecento) Es La più importante invenzione del Novecento/ventesimo secolo é... (The most important invention of the 20th century is....) (It's like the forties, the sixties, the seventies in English about years)

Because the XVI century is from 1500 till 1600, the XX century is from 1900 till 2000 etc etc ..

Millecinquecento-1600 is abbreviated in cinquecento. Then the talk about 500-600, we say 'quinto secolo' [dopo Cristo]; ambiguities might happen but actually quite seldom. VI century : Sesto secolo [dopo Cristo]. [dopo Cristo] is an option, depending upon the context and the possibility of ambiguous meaning.

A century goes from 1 to 100, extremes implied, so "Il XVI secolo" is from 1501 to 1600. The year 1600 is still XVI century. It isn't so named only in Italian, but in other European languages too but, il Cinquecento, il Seicento and so on, must be considered in the context (historical, scientific, philosophic, artistic etc. one) because usually those limits aren't so strictly defined. It isn't "sedici" because ordinal numbers are used (and not only in Italian). Have you ever read "In the first century A.D/B.C."/Nel primo secolo..." or similar sentences ? V = quinto secolo VI = sesto secolo etc.