In Italian, both the indefinite and the definite article change in spelling and pronunciation depending on the following sound, in the masculine gender.
Before vowels, the masculine indefinite article is un and the definite article is l' (elision of lo):
- un albero (a tree), l'albero (the tree)
Before single consonants or consonants followed by semivowels or liquids, the masculine indefinite article is un but the definite article is il, and this includes the affricate consonants /t͡ʃ/ (spelled 'c' before 'i' or 'e') and /d͡ʒ/ (spelled 'g' before 'i' or 'e'):
- un ramo (a branch), il ramo (the branch)
- un fiore (a flower), il fiore (the flower)
- un treno (a train), il treno (the train)
- un cielo (a sky), il cielo (the sky)
- un gioco (a game), il gioco (the game)
Before double consonants (generally 's' followed by a consonant, but other combinations arise in loanwords, especially of Greek origin, including with the double consonant 'x'), and before 'z' which is pronounced as the affricate /t͡s/ or /d͡z/, the masculine indefinite article is uno and the definite article is lo:
- uno spazio (a space), lo spazio (the space)
- uno psicologo (a psychologist), lo psicologo (the psychologist)
- uno xilofono (a xylophone), lo xilofono (the xylophone)
- uno zaino (a backpack), lo zaino (the backpack)
Feminine articles are una (indefinite) and la (definite), but they get elided into un' and l' before vowels:
- una foglia (a leaf), la foglia (the leaf)
- un'ombra (a shadow), l'ombra (the shadow)
These changes are not dependent on the noun the article connects to, but the sound immediately following it, so for example we have:
- l'albero (the tree), but il grande albero (the big tree)
- un gioco (a game), but uno speciale gioco (a special game)
- uno zaino (a backpack), but un piccolo zaino (a small backpack)
- una foglia (a leaf), but un'altra foglia (another leaf)
- un'ombra (a shadow), but una cupa ombra (a dark shadow)
These behaviors can be at least partly explained by the phonotactics of Italian: whenever uno is used, the phonotactic constraints wouldn't allow for un unless the 'n' were dropped entirely; the same issue gives rise to lo instead of il, which look very different on the surface, but both come from different parts of the Latin word illum or illud (a demonstrative).
The elided forms with an apostrophe can be explained by a tendency to eschew hiatus.