a + lo = allo (before m.sg nouns starting with s+consonant, z, gn, ps, etc.). a + l' = all' (before singular nouns starting with a vowel, either gender).
'allo' and 'all'' — a + lo / a + l'1 / 10
Vado ___ sportello bancomat. (I go to the ATM.) — 'sportello' starts with 'sp'